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Thgi Natural Histoiy o

Enewetak Atoll

DOeEV/06703-T1-Vbl. 1
(DE87006110)

Volume I The Ecosystem: Environments,
Biotas, and Processes

United states
Department of Energy

Office of
Energy Research

Office of Health and
Environmental Research

Ecological Research
Division

>»:k'

■h

Volume I

The Ecosystem: Environments, Biotas, and Processes

o«

? J)
■ o

Top: Aerial vit-w of Enewetak Atoll from an altitude of 10.000 ft looking north. The wide south passage to the lagoon is at the bottom of the pic- ^
tiirc. The three islands to the right of the passage are Enewetak, Medren, and Japtan. The deep east pass is seen between Medren and Japtan. Thei
five southwest islands are seen to the left of the wide south passage. Ikuren is the first one. North of these islands is the shallow southwest pass. The
Aloll is elliptical in shape measuring about 41 km from north to south and 33 km from east to west. [Photography by P. L. Colin.]

H<>lti>ni: Aerial view of the northern end of Enewetak Island showing the cluster of buildings of the Mid-Pacific Research Laboratory. The quarry
is visible on the reef flat. The small island immediately to the north is Bokandretak. [Photography by E. S. Reese.]

DOE/EV/00703-TI-Vol. 1
(DE87006110)

The Natural History of

Enewetak Atoll

/9 3

V. I

Volume I The Ecosystem: Environments, Biotas, and Processes

Edited by:

United States
Department of Energy

Office of Energy Research

Office of Health and
Environmental Research

Ecological Research Division

Prepared by

Office of Scientific and Technical Information
U.S. Department of Energy

^^tKWOG,^

*°

Dennis M. Devaney
Bernice P. Bishop Museum
Honolulu, Hawaii

Ernst S. Reese
University of Hawaii
Honolulu, Hawaii

Beatrice L. Burch

Bernice P. Bishop Museum

Honolulu, Hawaii

Philip Helfrich
University of Hawaii
Honolulu, Hawaii

MARINE
BIOLCGlCAl
LABORATORY

LtSR/ HY

V,t)ODS HCLl ma::
W. H. 0 I.

><-CW-i|J

[Photograph by Annabelle Lyman.]

Bok in, kon menninmour ko im menin eddok ko

ion Enewetak, ej kein kememej im kautiej ri Enewetak.

This volume on the natural history of Enewetak Atoll
is dedicated to the people of Enewetak.

International Copyright,® U. S. Department of Energy, 1987, under the provisions of the Universal
Copyright Convention. United States copyright is not asserted under the United States Copyright
Law, Title 17, United States Code.

Library of Congress Cataloging-in-Pablication Data

The natural history of Enewetak Atoll.

DOE/EV00703T1-Vol. 1 (DE87006110).

DOE/EV00703-Tl-Vol II (DE87006111).

Includes indexes.

1. Natural history — Marshall Islands — Enewetak
Atoll 1 Devaney. Dennis M II. United States.
Dept. of Energy Office of Scientific and Technical
Information

QH198,E53N38 1987 508 96'83 87-24863
ISBN 0-87079-5791 (set)
ISBN 0-87079-580-5 (pbk : set)
ISBN 0-87079-581-3 (microfiche : set)

Work performed under contract No. DEAC08-76EV00703

The set of two volumes is available as DE87006110 (DOE/EV/0073-Tl-Vol 1) and DE87006111

(DOE/EV/0073-Tl-Vol.2) from

NTIS Energy Distribution Center

P. O. Box 1300

Oak Ridge, Tennessee 37831

Price Code: Paper Copy A99
Microfiche AOl

Printed in the United States of America

1987

Foreword

As activity and funding at the Mid-Pacific Research Laboratory began to diminish in the early 1980s,
it seemed fitting that a synthesis be prepared of the three decades of research that had been conducted at
this Laboratory on Enewetak Atoll. For 30 years the Atoll served as a convenient, accessible location for
studies of Mid-Pacific island ecosystems, and several hundred scientists utilized the facility. Primary fund-
ing was provided by the Office of Health and Environmental Research, Ecological Research Division,
U. S. Department of Energy (formerly the Atomic Energy Commission and the Energy Research and
Development Administration).

This is an attempt to synthesize in two volumes the results of the Mid-Pacific Research Laboratory
studies that have been published in hundreds of widely dispersed publications. It is hoped that present and
future scientists involved in studies of Mid-Pacific islands will find this synthesis a convenient resource for
their research.

Considerable time and effort were expended by many contributors to make this synthesis [X)ssible.
Thanks are extended to all these authors for their manuscripts. Special appreciation is expressed for
Dr. Dennis Devancy's dedication in filling gaps in the taxonomic descriptions of several invertebrate
groups. This publication would not have been possible, however, without the determination and persis-
tence of Dr. Ernst Reese in organizing and collecting the material. Deepest gratitude is acknowledged for
his conscientious efforts.

Helen M. McCammon, Director
Ecological Research Division
Office of Health and Environmental Research
United States Department of Energy

Acknowledgments

Many people have contributed in many ways to the production of these two volunncs. Regardless of
the nature of the contribution, everyone listed below has given thought and time, that most precious com-
modity of thinking individuals, to bring The Natural Histor\^ of Enewetak Atoll into publication. Authors of
the chapters are not listed separately, even though, in most cases, they critically read other chapters. No
doubt we have overlooked many who have contributed in important ways, and for these oversights we
apologize. To all of you we wish to extend our deepest thanks.

Donald P. Abbott
Isabella A. Abbott
Hazel K. Asher
George H. Balazs
Jerry L. Barnard
Frederick M Bayer
Henry R. Bennett
Richard A. Boolootian
Thomas E. Bowman
Harry U. Brown
Fenner A. Chace, Jr.
G. Arthur Cooper

Edward B. Cutler
Mae G. De Rego
Maxwell S. Doty
Iraneus Eibl-Eibesfeldt
Robert E. Elbel
William O Forster
Vicki S. Frey
John T. Harrison
Janet F. Heavenridge
Derral Herbst
Robert W. Hiatt
Lipke B. Holthuis

Richard Houbrick
Arthur Humes
Edwin Janss
Robert E. Johannes
Victor R. Johnson, Jr.
Irene D. Keller
Phillip B. Lamberson
Jere Lipps
Frangois Mautin
Helen M. McCammon
Ellen Moore
William Newman

Jeri-Lyn Palacio
David Pawson
William F. Perrin
Marian Pettibone
Colin S. Ramage
Anita Savacool
Hajo Schmidt
Stephen V. Smith
Sy Sohmer
William J. Stanley
Lyn Sweetapple
Lori N. Yamamura

Preface

The two volumes of The Natural History/ of Enewetak
Atoll summarize research done at the Mid-Pacific Research
Laboratory from 1954 to 1984 under the auspices of the
Department of Energy. The history of the laboratory and
the reasons for its support by the United States Depart-
ment of Energy are described in Chapter 1 of Volume 1 .

Over a thousand persons — established scientists, their
assistants, and graduate students — conducted research at
the laboratory during the 30-year period. Their efforts
resulted in 223 publications. These have been collected in
four volumes of reprints entitled Mid-Pacific Marine Labora-
tory Contributions. 1955-1979, U. S. Department of
Energy, Publication NVO 628-1. The laboratory has con-
tinued operation on a limited scale to the present. A col-
lection of papers recently appeared in the Bulletin of
Marine Science, Volume 38, 1986.

Much of the research conducted at the laboratory was
on the marine environment. The reason was that the
majority of scientists applying to work at Enewetak were
marine biologists. For many, this was the first opportunity
to study the biota of a coral atoll. Fewer studies were con-
ducted in the terrestrial environment and its biota.
Nevertheless, as these volumes attest, the coverage is
amazingly complete and thorough, and there are few, if
any, studies of an equivalent ecosystem that equal the
total research effort reported in these volumes.

Volume I provides a synthesis of the research carried
out under the subject headings of the respective chapters.
Certain of the chapters, e.g., those on geology, subtidal
and intertidal environments and ecology, and those on reef
processes and trophic relationships, summarize a great
diversity of research carried out by many scientists for
many years. In contrast, the chapters on meteorology and
oceanography summarize research carried out under one
integrated program involving fewer scientists working over
a shorter period.

Volume II of The Natural Historic of Enewetak Atoll
provides information on the taxonomy of animals and
plants known to occur at Enewetak Atoll. This taxonomy
represents a fulfillment of one of the first assignments to
the laboratory — to determine the scientific names of the
biota of the atoll. The collections on which the checklists
in each chapter are based are housed at the Bcrnice P.

Bishop Museum in Honolulu and the U. S. National
Museum of Natural History, Smithsonian Institution, Wash-
ington, D. C.

In addition to the sp>ecies checklists, each chapter in
Volume II provides a succinct summary of the biota with
respect to endemism, range extensions, and other features
that set the Enewetak biota apart from those one might
expect to find on equivalent Indo-Pacific islands. This com-
pendium of taxonomic information for an atoll should
prove of immense value to scientists interested in biogeog-
raphy and evolutionary biology of island ecosystems for
years to come.

One of the problems of editing these volumes has been
the correct use of place names. In some cases authors
used the military code names for islands while others used
the native names. Even the native names have changed
from early phonetic spellings to the sp>ellings currently in
use and preferred by the Enewetak people. For example,
the name of the atoll has changed from Eniwetok to
Enewetak, and, although the correct current sf>elling is
used throughout, the old spelling occurs in older references
and maps which appear in these volumes. Maps giving the
military code names and the native names preferred by the
Enewetak people are located in Chapter 1 of Volume I.
Surprisingly, it is difficult to determine the exact number of
islands. Due to the effects of storms, small islands are
ephemeral, and two islands and part of a third were ob-
literated by nuclear explosions. Currently there arc 39 rec-
ognizable islands, and these are shown on the map used
throughout the book.

These volumes do not report on the extensive radiolog-
ical surveys and studies which have been conducted by the
Lawrence Livermore Laboratory, University of California,
and the Radiation Laboratory, University of Washington,
also under the auspices of the U. S. Department of
Energy.

Dennis M. Devaney, senior editor of this volume, disaf)-
peared while collecting specimens off the Island of Hawaii
on August 13, 1983. Dennis was doing what he loved
best, collecting marine invertebrates, at the time of his
death. He collected extensively at Enewetak, and he under-
took the task of organizing the systematic chapters of
Volume II. Beatrice L. Burch, Dcvaney's assistant at the

Bishop Museum, completed the task, and she has written of human history. In a small way, this book stands as

the introduction to Volume II. something good that has resulted from those years.

It is fitting that the two volumes of this book are dedi
cated to the people of Enewetak Atoll. They, like so many Ernst S. Reese

other human beings, were caught up by forces beyond Professor of Zoology

their control and understanding in an immense cataclysm University of Hawaii, Honolulu

Contributors

Marlin J. Atkinson

University of Western Australia, Nedlands, Australia

Robert K. Bastian

U. S. Environmental Protection Agency, Washington, D.C.

Andrew J. Berger

University of Hawaii, Honolulu, Hawaii

Patrick L. Colin

University of Papua New Guinea, Port Moresby,
Papua New Guinea

Robert A. Duce

University of Rhode Island, Kingston, Rhode Island

Ray P. Gerber

St. Joseph's College, North Windham, Maine

Philip Helfrich

University of Hawaii, Honolulu, Hawaii

William B. Jackson

Bowling Green State University, Bowling Green, Ohio

Robert C. Kiste

University of Hawaii, Honolulu, Hawaii

Alan J Kohn

University of Washington, Seattle, Washington

James A. Marsh, Jr.

University of Guam, Mangilao, Guam

Nelson Marshall

University of Rhode Island, Kingston, Rhode Island

John T. Merrill

University of Rhode Island, Kingston, Rhode Island

Roger Ray

U.S. Department of Energy, Las Vegas, Nevada

Ernst S. Reese

University of Hawaii at Manoa, Honolulu, Hawaii

Byron L. Ristvet

S-CUBED, Division of Maxwell Laboratories,
Albuquerque, New Mexico

Stephen H. Vessey

Bowling Green State University, Bowling Green, Ohio

Contents

Chapter Pagg

Introduction ^^■^^

Ernst S. Reese

1 Research at Enewetak Atoll: A Historical Perspective 1

Philip Helfhch and Roger Ra\^

2 History of the People of Enewetak Atoll 17

Robert C Kiste

3 Physiography of Enewetak Atoll 27

Patrick L. Colin

4 Geology and Geohydrology of Enewetak Atoll 37

Bi^ron L. Ristuet

5 Oceanography of Enewetak Atoll 57

Marlin J. Atkinson

6 Meteorology and Atmospheric Chemistry of Enewetak Atoll 71

John T. Merrill and Robert A. Duce
1 Subtidal Environments and Ecology of Enewetak Atoll 91

Patrick L. Colin

8 Intertidal Ecology of Enewetak Atoll 139

Alan J. Kohn

9 Reef Processes: Energy and Materials Flux 159

James A. Marsh. Jr

10 Trophic Relationships at Enewetak Atoll 181

Nelson Marshall and Rav P. Gerber

11 Terrestrial Environments and Ecology of Enewetak Atoll 187

Ernst S. Reese

12 Biology of the Rodents of Enewetak Atoll 203

Williann B. Jackson. Stephen H. Uessey, and Robert K. Bastion

13 Avifauna of Enewetak Atoll 215

Andrew J. Berger

Author Index 221

Subject Index 223

Introduction

Ernst S. Reese

University/ of Hawaii at Manoa
Honolulu. Hawaii 96822

The first volume of The Natural Histori/ of Enewetak
Atoll provides a summary of the research carried out over
the 30-year period from 1954 to 1984. The frontispiece
illustrates the dramatic contrasts between the immensity of
the lagoon and the seemingly fragile necklace of small
islands which surrounds it, and also between the sea condi-
tion on the windward, seaward side of the reef and the
relatively sheltered waters of the lagoon.

The first chapter discusses the history of research at
Enewetak Atoll. The reasons behind the establishment of
the Enewetak Marine Biological Laboratory are described.
The authors, Philip Helfrich and Roger Ray, have been
associated with activities at Enewetak from the very early
days. They conferred with Robert W. Hiatt, the first direc-
tor of the laboratory. In Chapter 2, Robert C. Kiste, a
foremost authority on the f)cople of Micronesia, provides a
history of the Enewetak people to whom these volumes
are dedicated.

The next four chapters deal with the physical environ-
ments of Enewetak Atoll. In Chapter 3, Patrick L. Colin
describes the physiography of Enewetak. Colin served as
resident scientist in charge of the laboratory from 1979 to
the end of 1983 when all resident scientific staff left the
atoll. Following the description of the atoll, Byron L. Rist-
vet, a frequent scientific visitor to Enewetak, provides a
summary of the geology and geohydrology in Chapter 4.
Next, in Chapter 5, Marlin J. Atkinson describes the
oceanography. Under the direction of Stephen V. Smith,
Atkinson participated in an important study of the lagoon
circulation. Chapter 6 on the meteorology and atmos-
pheric chemistry is the final chapter in the group of
chapters dealing with the physical environment of
Enewetak Atoll. Written by John T. Merrill and Robert A.
Duce, the chapter is based on the results of the SEAREX
Project. Duce served as the director and principal investi-
gator of the project.

The next four chapters are devoted to the marine
ecosystem and its biota. They summarize the large amount
of research carried out at the Mid-Pacific Research Labora-
tory in the marine environment. All of the authors were

frequent visitors to the laboratory, and they have done a
splendid job of reviewing the research carried out in their
area of interest. In Chapter 7, Patrick L. Colin describes
the subtidal environments of Enewetak and reports on the
research done on the subtidal biota. This is followed in
Chapter 8 by Alan J. Kohn's masterful summary of
research in the intertidal environment. Kohn has been a
student of tropical intertidal ecology for 30 years. He tack-
led a particularly difficult task because of the extensive
study of the intertidal environment and its biota by many
scientists over the years.

Chapters 9 and 10 deal with processes and relation-
ships in the marine environment. In Chapter 9, James A.
Marsh, another frequent visitor to the laboratory and a
recognized authority on coral reef processes, reviews the
extensive work which was carried out at Enewetak on the
community metabolism of coral reefs and related topics
such as calcification processes, nitrogen and phosphorus
cycles, and the role of detritus in the ecosystem. Nelson
Marshall and Ray P. Gerber extend the ecosystem
approach in Chapter 10 to include the entire atoll. They
discuss the trophic relationship between the shallow reefs
and the lagoon. Both Gerber and Marshall conducted
research at Enewetak.

The final three chapters are devoted to the terrestriEd
environment. Because fewer scientists applied to conduct
research in the terrestrial environment, less work was
accomplished, and an integrated overview is not possible.
In Chapter 11,1 rep>ort on the life history, behavior, and
ecology of land crabs, review what is known about atoU
soils, and conjecture on the carrying capacity of an atoll
such as Enewetak. For a description of the vegetation, the
reader is referred to Chapter 3 in Volume II by Janet O.
Lamberson. William B. Jackson, a frequent visitor to
Enewetak over the years, and his co-workers Stephen H.
Vessey and Robert K. Bastian report on their long-term
study of the rodents in Chapter 12, and Andrew J. Berger
summarizes our knowledge of the bird life of the atoll in
Chapter 13. Berger, a noted ornithologist and the
foremost authority on Hawaiian birds, made a number of
trips to Enewetak.

1 suspect that few readers will read this volume from
cover to cover, but those who do will gain an appreciation
for the complexity of the atoll ecosystem and a better

XVII

understanding of the intimate relationships between the
seemingly fragile components of the ecosystem: the
lagoon, the reefs, the islands and their biotas, all perched
on a volcanic and coral pinnacle in the vastness of the
Pacific Ocean. In the final analysis, however, the book will

serve its purpose best if the reader comes away with more
questions than answers and a desire to find the answers to
these questions in future research on the natural history of
coral reefs and islands.

Chapter 1

Research at Enewetak Atoll: A Historical Perspective

PHILIP HELFRICH' and ROGER RAYf

'Hawaii Institute of Marine Biologi^. Uniuersiti/ of
Hawaii, Kaneoke. Hawaii 96744:
fNevada Operations Office, U. S^ Department of
Energi^, Las Vegas, Nevada 89114: current
address is 10252 Hatherleigh Dr., Bethesda.
Maryland 20814

INTRODUCTION

The Pacific theater of operations in World War II
brought millions of military personnel to the tropical
Pacific, and their activities on the Pacific Islands afforded
close contact and awareness of the physiography and
natural history of these small dots of land scattered in the
vast expanse of ocean. This enhanced awareness, coupled
with a recognized need by the military establishment for
increased knowledge of Pacific Island areas, led to
government-sponsored investigations, complemented by
efforts of many individual scientists whose interest had
been stimulated by wartime visits to these islands. In the
postwar period, two activities of the U. S. government
focused further interest on the coral atoll of the tropical
Pacific and influenced the future of research at Enewetak
Atoll (Figs. 1 and 2). The origin of the spelling "Eniwetok"
is lost but would appear to be a phonetic rendering of
what the people called their atoll. In 1973 it gave way to
the current spelling, consistent with written Marshallese,
and meaning "island which points to the east."

World War II demonstrated the importance of these
small, scattered land masses to any military confrontation
in the Pacific basin. After the war, the U. S. Navy moved
to develop a series of permanent bases from among the
many temporary wartime bases and outposts which had
been established across the Pacific. With the prominent
role of the Navy in developing and maintaining these
bases, it is not surprising that the Navy's research arm,
the Office of Naval Research (ONR), inaugurated a scien-
tific program in the late 1940s aimed at a better under-
standing of atoll morphology and of all aspects of island
life from microorganisms to human inhabitants. The ONR
funded a series of expeditions in conjunction with the
Pacific Science Association, many of which were to atolls

in the central and western tropical Pacific. Arno Atoll in
the southern Marshall Islands and Onotoa Atoll in the Gil-
bert Islands (now Kiribati) were subjects of intensive inves-
tigation in 1950 and 1953, respectively. Scientists
involved in these atoll studies contributed to the establish-
ment of the Eniwetok Marine Biological Laboratory (EMBL)
on Medren Island, Enewetak Atoll, in 1954.

The second postwar activity which served to focus
attention on the mid-Pacific area was the atomic wcafwns
testing program in the northern Marshall Islands. Two
atomic weapons had inflicted mortal damage upon Japan
and had brought a precipitous end to the war in the
Pacific. Military planners and strategists knew very little
about this new and awesome strategic resource. Thus, an
area was sought which might accommodate full-scale test-
ing of atomic weapons. Neil Mines (1962) in his book
Proving Ground describes the process of choosing the
northern Marshall Islands as the testing site. First Bikini
Atoll and then Enewetak Atoll became test sites, to be
known together as the Pacific Proving Ground. National
security considerations soon led to research and develop-
ment testing and, with the impetus of the cold war, to the
testing of thermonuclear weapons in these islands. In all,
between 1946 and 1958, 43 nuclear devices were tested
at Enewetak and 23 on Bikini — events which were to have
profound and lasting environmental, social, and cultural
effects upon these two atolls as well as others nearby. The
nuclear testing program provided a setting, a focus of
interest, and an opportunity for research in the northern
Marshall Islands which eventually led to the establishment
of the EMBL.

THE WEAPONS TESTING PROGRAM

Soon after the 1946 tests at Bikini (Operation
Crossroads), which had been designed to assess the mili-
tary significance of atomic weapons, the United States
Congress created the Atomic Energy Commission (AEC), a
civilian agency charged with responsibility for the research,
development, testing, and production of nuclear weapons.
This new agency was to become host and manager of the
Pacific Proving Ground and, later, sponsor of EMBL.

Operation Crossroads was largely a seaborne opera-
tion, with logistic support from the naval base at

HELFRICH AND RAY

-^LJ

^y^JAPAN

yiOVAr

-A.AHAN ^^
ISLANDS W

MAft)ANA5

•WAKE

ISLANDS

JOHNSTON

GUAM.

ENFWFTAK ATOLL ^

UJELANG
ATOU _^

Af BIKINI ATOLL

* -'■ MAffSHALL
- '. I5LAM0S
KWAJALEIH^^ ■ >

. .. -

■■6 ; ,

• .

CAftOLINE ISLANDS

■ 51LBC«T

' ISLANDS

^CAHTON ISLAND ' ,^^,5,^,,

» 'SlanD

N€* CUINCA^"'"^,

SOLOMON

.^,,s..s

REGIONAL MAP

CMaPhiC SCAI.E IN •«Aotm:ai. wilCS

^jMIOWAY

,0^

_ioii.^ —

^~"^"«f

-'-"^HONOLULU

,-''"/■*«(

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,»>v^

---^ItXNSTON

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— — i°-'0___^^^ENEWETAIt .

AIRLINE DISTANCES MAP

NO SCALE

Fig. 1 Regional and airline distances maps of the Pacific and tlie Marshall Islands showing
location of Enewetak Atoll.

Kwajalein. It consisted of two tests, one an airdrop and
the other an underwater detonation. The radiation and
other effects of both of these tests — code-named Abie and
Baker — were largely confined to Bikini Atoll, with such
fallout as left the Bikini area being deposited in areas of
open ocean. The same could be said of the early develop-
ment tests, which began at Enewetak in 1947. The selec-
tion of these atolls had been strongly influenced by their
remoteness and by the predictability of wind conditions.

The 1954 operation, code-named Castle, was planned
contemplating use of both atolls. Detonation of Bravo, the

first test of Castle, drastically altered that plan. The explo-
sive power (yield) of Bravo was more than twice that
which had been predicted, and local winds carried the
debris, or local fallout, directly across Bikini Atoll, contami-
nating much of the land area and rendering the control
area and many of the experimental sites unusable for the
remainder of the Castle operation (Hines, 1962). Some
testing continued at Bikini, but Enewetak, after Bravo,
took on even greater importance in the atmospheric
nuclear testing program. During the period which ended on
October 31, 1958, Enewetak was the site of 43 nuclear

A HISTORICAL PERSPECTIVE

10 MILES

Fig. 2 Islands of Enewetak Atoll with Marshallese names shown on the lagoon side and English
code names on the ocean side.

weapon tests. Enewetak, Medren, and Japtan Islands
housed the command, administrative, logistic, and techni-
cal support facilities, and the islets in the northern and
eastern portions of the atoll served as test areas. Table 1
lists the detonations at Enewetak, and Fig. 3 illustrates the
test locations on the atoll.

The nuclear testing program required the mobilization
of a vast assemblage of scientists, technicians, and support
personnel and the establishment of laboratories, shops, and
living quarters, in addition to port facilities and an air ter-
minal to connect with a supply system extending through
Hawaii to mainland bases as far as 8000 miles away. Test

operations over more than a decade were conducted by a
series of Joint Task Forces (JTFs), consisting of Army,
Navy, Air Force, and AEC elements, in a coordinated
operational command. The commander was a senior mili-
tary officer of flag rank and had as his deputy a senior
AEC scientist.

The test detonations were grouped in series which,
typically, lasted several months. During the times between
series — usually a year or more — the support apparatus
continued to function. This availability of logistic and
administrative support made it feasible to consider the
establishment of a laboratory facility. The AEC interest in

HELFRICH AND RAY

TABLE 1
Nuclear Tests at Enewetak Atoll

Operation

Type and

event name

Date

height, ft

Yield

Location

Sandstone

X-ray

4/14/48

Tower 200

37 KT

Janet, west tip

Yoke

4/30/48

Tower 200

49 KT

Sally

Zebra

5/14/48

Tower 200

18 KT

Yvonne, north end

Greenhouse

Dog

4/7/51

Tower 300

Yvonne, north end

Easy

4/20/51

Tower 300

47 KT

Janet, west tip

George

5/8/51

Tower 200

Ruby

Item

5/24/51

Tower 200

Janet, north tip

Ivy

Mike

10/31/52

Surface

10.4 MT

Flora

King

11/15/52

Airdrop 1500

500 KT

Yvonne, 2000' N

Castle

Nectar

5/13/54

Barge

1.69 MT

Mike Crater

Redwing

Lacrosse

5/4/56

Surface

40 KT

Yvonne, north end

Yuma

5/27/56

Tower 200

Sally, west tip

Erie

5/30/56

Tower 300

Yvonne, by airstrip

Seminole

6/6/56

Surface

13.7 KT

Irene

Blackfoot

6/11/56

Tower 200

Yvonne, middle

Kickapoo

6/13/56

Tower 300

Sally, north tip

Osage

6/16/56

Airdrop 670

Yvonne, middle

Inca

6/21/56

Tower 200

Pearl

Mohawk

7/2/56

Tower 300

Ruby

Apache

7/8/56

Barge

Mike Crater

Huron

7/21/56

Barge

Mike Crater

Hardtack, Phase I

Cactus

5/5/58

Surface

18 KT

Yvonne, north end

Butternut

5/11/58

Barge

Yvonne, 4000' SW

Koa

5/12/58

Surface

1.37 MT

Gene

Wahoo

5/16/58

Underwater 500

James, 7400' S

Holly

5/20/58

Barge

Yvonne, 2075' SW

Yellowwood

5/26/58

Barge

Janet, 6000' SW

Magnolia

5/26/58

Barge

Yvonne, 3000' SW

Tobacco

5/30/58

Barge

Janet, 4000' SW

Rose

6/2/58

Barge

Yvonne, 4000' SW

Umbrella

6/8/58

Underwater 150

Glenn, 7400' N

Walnut

6/14/58

Barge

Janet, 6000' SW

Linden

6/18/58

Barge

Yvonne, 2000' SW

Elder

6/27/58

Barge

Janet, 4000' SW

Oak

6/28/58

Barge

8.9 MT

Alice reef, 3 mi SW

Sequoia

7/1/58

Barge

Yvonne, 2000' SW

Dogwood

7/5/58

Barge

Janet, 4000' SW

Scaevola

7/14/58

Barge

Yvonne, 561' SW

Pisonia

7/17/58

Barge

Yvonne, 12000' W

Olive

7/22/58

Barge

Janet, 4000' SW

Pine

7/26/58

Barge

Janet, 8500' SW

Quince

8/6/58

Surface

Yvonne, middle

Fig

8/18/58

Surface

Yvonne, middle

A HISTORICAL PERSPECTIVE

162-10'E

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Fig. 3 Enewetak Atoll nuclear tests with name, year of detonation, and approximate locations.

expanding knowledge of the environmental setting in which
the tests were being conducted provided the basis for
discussions which led to the establishment of the EMBL.

ESTABLISHMENT OF EMBL

Of necessity, the nuclear testing program of the 1940s
and 1950s was conducted in a climate of national urgency
and classification security. Important scientific and strategic

information had been lost to foreign powers in the immedi-
ate postwar period, and the pace of atomic weapons
research and development had become a vital indicator of
political power. In this environment, the establishment of a
university-associated research laboratory, with its traditions
of academic freedom and open publication of research
results, was nothing less than remarkable. It reflected the
enlightened scientific climate of the AEC and the AEC's
concern regarding the long-term consequences of applica-

HELFRICH AND RAY

tions of nuclear technology. There was a need for more
complete knowledge of the dynamic biogeochcmical
processes which might lead to the transp)ort of radioactive
contaminants in the atoll system to man. More fundamental
was the acknowledged inadequacy of our understanding of
the systematics and ecology of the highly diverse atoll
biota. Early records of environmental monitoring during the
test series included entries such as "red fish" and "green
filamentous algae," reflecting the lack of any pertinent tax-
onomic descriptions of the local biota. The College of
Fisheries of the University of Washington, under contract
to the AEC, had conducted studies at Bikini and Enewetak
of the interaction of environmental radioactivity with vari-
ous species and had made substantial contributions to the
literature regarding these nuclear-affected atolls (Mines,
1962). There remained, however, a need for a broader
base of information about the systematics, ecology, and life
history of the atoll flora and fauna.

Details of the discussions leading to the establishment
of EMBL are unavailable. In the early 1950s, however, the
eminent biologist, H. Burr Steinbach, then of the Univer-
sity of Chicago and later of Woods Hole Oceanographic
Institution, was asked by Sidney Caller of the Office of
Naval Research to travel to Enewetak Atoll to explore the
feasibility of establishing a marine biological laboratory.
Steinbach's trip and his subsequent report recommending
the establishment of a laboratory on Enewetak Atoll were
instrumental in AEC's action to contract with the Univer-
sity of Hawaii to establish and operate the EMBL.

The contract, signed on June 3, 1954, required the
university to manage the laboratory and to direct and coor-
dinate its scientific programs. Policy direction and sponsor-
ship were provided "by the Division of Biology and Medi-
cine of the AEC Headquarters in Washington, D. C.
Robert W. Hiatt, Director of the Hawaii Marine Labora-
tory, became the first director of EMBL. The first orders
of business were to provide supplies, equipment, and work
areas for visiting investigators and to establish a reference
collection of animals and plants with an ecological index
for their use.

To facilitate scientific investigations of terrestrial and
intertidal biota, two islets on Enewetak Atoll — Ikuren and
Mut — were set aside as reserves for the exclusive use of
EMBL scientists. This was done to ensure that a continu-
ously available source of typical fauna and flora would be
protected, to the extent possible, from proving ground
activities. During these early years, EMBL scientists were
permitted to use the laboratory only in the intervals
between test series. However, marine scientists from the
University of Washington Applied Fisheries Laboratory,
under separate contract to the AEC, were in residence
during the actual test events. Their work at Enewetak and
elsewhere in the Pacific is recounted by Hines (1962) and
is reported in numerous published papers.

The laboratory was first quartered in a rectangular
metal building, with an aquarium lanai, located on the
southwest shore of Medren Island. The building was
equipped with a simple seawater system, a single air-

conditioned instrument room containing microscopes, a
small library, and an assortment of nets, diving gear, and
other field equipment. Being a sponsored tenant in the
proving ground — which in peak periods accommodated
hundreds of scientists, technicians, and supp)ort per-
sonnel— the laboratory enjoyed superb facilities for dining,
housing, recreation, and medical care.

During the 1950s, 1960s, and early 1970s, the labora-
tory was operated on a part-time basis, with the active
periods generally dictated by university class schedules.
Thus, most investigators visited during the summer months
and the periods of winter or spring academic holidays.
Also during this period, visit authorizations were restricted
to male U. S. citizens who had passed a security screen-
ing. Travel to Enewetak from Honolulu was by military or
military charter aircraft. The flight time from Honolulu to
Enewetak was about 10 hours, usually with stops at John-
ston Island and at Kwajalein and/or Wake Island. It is
noteworthy that, despite considerable resistance to the
invasion by women of what had been traditionally an
exclusively male territory, arrangements were made to
accommodate the eminent zoologist E. Alison Kay at the
Enewetak Laboratory in December 1970. Her arrival sig-
naled a new era in which the merits of the scientific
research proposed were the only criteria for acceptance of
a researcher at EMBL.

Initially, the research emphasis at EMBL was toward
the establishment of a reference collection of the local
marine flora and fauna. This was accomplished by special-
ists, who made extensive collections of particular groups of
animals and plants, identified the individual specimens
(including those new to science), labeled, cataloged and
preserved them, and placed them in the laboratory collec-
tion room. To complement the reference collection, a small
library was established on site, providing convenient access
not only to published references and texts but also to the
works, both published and unpublished, of visiting investi-
gators. Notices placed annually in the journal Science
served to call this facility and its superb atoll environment
to the attention of the community of marine scientists.
This early research and subsequent publicity regarding the
EMBL facility, combined with the availability of modest
research grants, brought an enthusiastic response. From
1954 until this writing, 1028 scientists have worked at
Enewetak, many returning for several periods of field col-
lection and investigation. Notable was the response of tem-
perate zone biologists who had not previously worked in
the tropics. Entering the strikingly clear lagoon waters for
the first time, with no more complex equipment than a
face mask, was an exciting experience. Examination of a
coral pinnacle, with its enormous diversity of organisms,
brought a whole series of new dimensions to the work of
these scientists. The limitations of the physical facilities
and the remoteness of the EMBL field station were offset
by an abundance of exciting research opportunities and vir-
tual freedom from the pressures and distractions of cam-
pus life. These features resulted in a level of scientific pro-
ductivity unequaled in the experience of most researchers.

A HISTORICAL PERSPECTIVE

The original EMBL building eventually proved inade-
quate to the needs of the scientists and in 1956 was
expanded to include an extension for storage and a
4' X 20' concrete tank to hold experimental animals.
Further expansion of the laboratory occurred in 1959
when Albert L. Tester of the University of Hawaii initiated
a major program in shark physiology and behavior. For
this program, two interconnected parallel tanks were con-
structed, which allowed sharks to swim in an oval pattern.
This facility permitted Tester and his colleagues to hold
and condition sharks, to test their reactions to various
chemical stimulae, and to elucidate some of the anatomical
and neurological bases for their aggressive behavior.

Nuclear testing activities at Enewetak ended in late
1958 with the declaration by President Eisenhower of a
moratorium (accompanied by a similar Soviet moratorium)
on all nuclear testing. The 1958 moratorium, originally a
1-year commitment, was actually continued until Sep*-
tember 1961. At that time, the Soviets suddenly resumed
testing at a high rate. Even then, however, the United
States, in its response, did not return to testing in the
Marshall Islands. Although the AEC continued to adminis-
ter the Pacific Proving Ground until it was transferred to
the Navy in 1960, AEC gradually withdrew activities and
support on Medren until EMBL was the only active facility
on that island. This made support such as power, water,
housekeeping and messing, and logistics difficult. In 1961
EMBL moved from Medren to Enewetak Island where an
active support infrastructure still existed. The laboratory's
new home became a building on the lagoon side of
Enewetak Island, previously used as a recreation center
(Figs. 4 and 5). This building was modified to provide two
small air-conditioned rooms for the protection of instru-
ments and chemicals. A rectangular aquarium was con-
structed in the center of the large main room which was
enclosed on three sides and open to the lagoon. A sea-
water system was installed, and living quarters were pro-
vided for EMBL personnel and visiting scientists in a build-
ing across the lagoon road from the laboratory complex.
Although adequate, this facility had one imp)ortant draw-
back. Boat operations required the use of the utility pier at
the northeast end of the island, making loading and
unloading difficult, and necessitating the carrying of equip*-
ment and specimens between the pier and the laboratory.
By 1969, another move was in order.

In this same year, the directorship of EMBL passed
first from Robert W. Hiatt to Vernon E. Brock, and then, a
few months later, to Philip Helfrich. Helfrich continued as
director until January 1, 1975.

In 1969, military activities at Enewetak dictated
another move for EMBL, this time to the vicinity of a
large, three-story dormitory building which had been con-
structed on the ocean side, toward the middle of Enewetak
Island. The new location was a complex of aluminum build-
ings, previously used as library, recreation center, and
darkroom. This location was more desirable because of its
proximity to sleeping quarters, food service facilities, and
the boat launching ramp. In addition, it included a large,

covered lanai — which was supplied with running seawater
for aquaria — and two portable swimming pools used as
holding tanks. With about twice the space that had previ-
ously been allocated, the new facility included a large gen-
eral laboratory, a shop, photo darkroom, library, equip-
ment room, communications room, a dive locker, and a
separate building for the storage of hazardous chemicals
(Fig. 6). In the early 1970s, EMBL acquired its own com-
munication system, providing a voice and teletype link to
the University of Hawaii.

MOVES TOWARD RESETTLEMENT

The year 1972 brought significant fxjlitical develof)-
ments which were to have a lasting effect upon the future
of the people of Enewetak and upon the fortunes of
EMBL. Political status talks had been going on for several
years between the government of the United States and
representatives of the people of the Trust Territory of the
Pacific Islands (TTPI). These talks were aimed at ultimate
termination of the United Nations trusteeship over the
Micronesian Islands (with the United States as trustee) and
the establishment of one or more new and independent
self-administering political entities. During the 1972 talks,
responding to the pleas of the people of Enewetak for the
return of their home islands, the United States took the
first steps toward that return. In April, Ambassador Hay-
den Williams, the President's personal representative to
the talks, was joined by High Commissioner Edward John-
ston of the TTPI in a public statement of U. S. intentions.
It provided that military use of Enewetak would shortly be
completed, thus permitting the atoll to be returned to the
administration of the Trust Territory, and that steps neces-
sary to rehabilitate the islands for resettlement could then
begin.

Later in 1972, the AEC's Nevada Operations Office,
using the resources of its national laboratories and contrac-
tors, mounted a massive radiological survey of Enewetak
Atoll as a preliminary step toward cleanup and rehabilita-
tion. These activities are described in official reports
(U. S. AEC, 1973; U. S. DOE, 1982; Holmes and
Narver, 1973; and U. S. DNA, 1975). Although EMBL
did not participate directly in either the 1972 survey or
the cleanup, the director and other scientists consulted and
assisted in many ways. While applied science and engineer-
ing were at work to restore the atoll, the basic studies of
EMBL continued apace. Although this tiny, remote
research station might have been overwhelmed by the
enormity of the cleanup effort (thousands of men, over 3
years, at a cost of more than $100 million), those respon-
sible in the AEC (now the U. S. Department of Energy)
and the U. S. Defense Nuclear Agency (DNA), recognized
the lasting worth of the science program and saw to it that
the laboratory's interests were protected.

In 1978, the U. S. Coast Guard LORAN Station,
which had occupied a complex of buildings at the eastern
end of Enewetak Island, was closed. By agreement with
DNA and with the p)eople of Enewetak, DOE obtained the

HELFRICH AND RAY

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A HISTORICAL PERSPECTIVE

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Fig. 5 The second laboratory facility was located on Enewetak Island from 1961 to 1969. [Photo by E. S. Reese.]

long-term use of these facilities and allocated them to the
laboratory (Fig. 7). Over the next 2 years, in anticipation
of the demobilization of the cleanup force and the sharp
reduction in available logistics and life support facilities,
steps were taken to make the laboratory ready to "stand
alone." The complex was augmented with several portable
housing and laboratory units, and plans were made for
local power, fresh and salt water systems, and other
needed support. The new location was a considerable
improvement, consolidating all operational and support
activities in one location. The new facilities included a
main air-conditioned laboratory building with work benches
and equipment space, a library, communications room,
dark room, reference collection roorr and several storage
rooms. Attached to the main building were a generator
room and a storage shed. Four additional buildings pro-
vided sleeping quarters accommodating as many as 18 per-
sons. Other buildings provided a kitchen, food storage, a
chemistry laboratory, a scientific shop, a dive locker, a
general maintenance shop, and a covered seawater lanai.

A 50-foot tower on which two 600-gal tanks were located
provided gravity feed for a seawater system. Good quality
unfiltered seawater for this system was pumped from a
former quarry in the reef.

Access to the lagoon for boats and personnel was pro-
vided by a conveniently located concrete ramp and a
wooden pier. Laboratory boats were moored offshore or
launched and retrieved from trailers at the ramp.

Fresh water was provided by catchment of rain from
the roofs of several buildings and stored in four 10,000-gal
cisterns. Diesel and gasoline fuels were stored in tanks on
the lagoon side of the laboratory complex. These fuels,
along with other supplies, were delivered to the laboratory
approximately every 2 months by the DOE research vessel
L\kianur, which was based at Kwajalein and supported
DOE's environmental research, radiation protection, and
medical programs in the northern Marshall Islands. Person-
nel, mail, and light cargo were usually transported via the
Airline of the Marshall Islands (AMI) on approximately a
biweekly schedule and occasionally on a chartered flight.

HELFRICH AND RAY

Fig. 6 The third laboratorv facility was larger and in a more convenient location on Enewetak
Island from 1969 to 1978. The name was changed to the Mid-Pacific Marine Laboratory (MPML) to
emphasize the broader research purview of the laboratory. [Photos by E. S. Reese.]

A HISTORICAL PERSPECTIVE

Fig. 7 The fourth and final location of the laboratory was in the former U. S. Coast Guard
LORAN Station on Enewetalt Island from 1978 to the present; a. The dormitory is to the left
and the mess hall to the right; b. View of the laboratory complex from the 50-ft-hlgh water
tower with one of the cisterns in the foreground. The name was again changed to the Mid-
Pacific Research Laboratory (MPRL) to note the inclusion of terrestrial as well as marine
research. [Photos by P. Helfrich.]

HELFRICH AND RAY

RESEARCH EMPHASIS

There were two major periods of research at Enewetak
conducted by the University of Hawaii under contract with
DOE and its predecessors. During the first 20 years (1954
to 1974), the AEC supported independent research that
was broadly aimed at increasing our knowledge of this rich
and diverse coral atoll ecosystem. The rationale for suf)-
porting this broadly based research was that it was impos-
sible to predict what aspects of the system might be most
perturbed by the test activities or what the lasting effects
of these perturbations might be. Thus, a broad spectrum
of investigations was considered appropriate. In retrosp>ect
this was a wise choice because later events and decisions
depended upon information resulting from this early
research. Scientists from EMBL, with their acquired data
base, were frequently called upon for advice and assis-
tance, especially during the period of preparation of the
atoll for the return of the Enewetak people. The modest
cost of maintaining and op)erating the laboratory over these
years provided the AEC with a bargain in science because
the support systems were in place for AEC and defense
department programs. The incremental cost of supporting
the laboratory was, therefore, relatively small. The scien-
tific research was accomplished at low cost because most
of the participating scientists were salaried by their home
institutions.

Much outstanding research was accomplished at EMBL
(Fig. 8). The record of accomplishment is set forth in the
volumes of collected reprints of scientific publications
which were issued in 1976 and 1979 (U. S. ERDA, 1976;
U. S. DOE, 1979). As knowledge of coral reef ecosystems
advanced, it was deemed advisable to mount a major
effort to understand the metabolism of an entire atoll
(Fig. 8). Discussions and planning conferences culminated
in the initiation of a major program in the summer of
1971 under the name SYMBIOS. This program lasted for
12 weeks and involved the research vessel Alpha Helix, 25
participating scientists, and numerous support (>ersonnel
under the leadership of Robert Johannes. SYMBIOS was
jointly sponsored by the National Science Foundation, the
AEC, and the Janss Foundation. Its initial objective — to
study the metabolism of an entire atoll — proved to be too
ambitious, but a thorough study of the windward reef was
accomplished and some major advancements were redized
in our knowledge of reef metabolism. As with other
research, this effort posed many new questions and chal-
lenges, and resulted in repeat visits to Enewetak by SYM-
BIOS scientists to further pursue work initiated in this
landmark study. The results of SYMBIOS are summarized
in Chapters 9 and 10 of this volume.

In 1972, the DNA began a series of studies to better
understand cratering effects of nuclear explosions. Craters
formed by the nuclear explosions of earlier years were
analyzed by direct observation, seismic response measure-
ments, and dynamic experiments utilizing chemical explo-
sives. Scientists from EMBL were called UF>on to advise the
defense department, especially upon the expected impact

of their experiments on the marine environment. Later, fol-
lowing a strong protest and legal action by lawyers for the
people of Enewetak, the dynamic experiments were can-
celed and only shallow coring of the atoll rim and seismic
studies of the reef structure were pursued to complete this
project.

The second period of research began with the reorgani-
zation of the laboratory in 1974. Following discussions
with the Chairman of the Atomic Energy Commission,
Dixie Lee Ray, a visit was made to the laboratory by an
ad hoc advisory group, including officials and scientists
from the University of Hawaii, the AEC, and several
indep>endent consultants. Chairman Ray had expressed an
interest in reorganizing and upgrading the laboratory to a
full-time oF>eration, with research objectives more directly
relevant to AEC interests. The advisory group met at
Enewetak in February 1974 and later made brief visits to
Bikini and to Majuro, the capital of the Marshall Islands.
Participants were William O. Forster, Nathaniel Barr, and
Charles Osterberg of AEC Headquarters; Roger Ray of the
Nevada Operations Office of the AEC; Philip Helfrich of
the University of Hawaii prector of EMBL); William R.
Coops of the Research Corporation of the University of
Hawaii; Robert Hiatt of the University of Alaska (first
Director of EMBL); and Glen Fredholm, an independent
consultant. The advisory group: (1) articulated in some
detail its recommended objectives for a laboratory agenda
which would be responsive to AEC direction, (2) suggested
that the field station at Enewetak be up>graded to full-time
activity with a resident staff, and (3) recommended that
the name of the laboratory be changed to the Mid-Pacific
Marine Laboratory (MPML) to reflect its interest in a wider
geographical area, including such areas as Bikini, where
the AEC continued to have an active interest.

In March 1974, following the advisory group meetings,
Roger Ray and Philip Helfrich returned to Majuro to meet
with officials of the government of the Marshall Islands and
with members of the Enewetak Municipal Council. The
latter meetings were hosted by Micronesian Legal Services
Corporation, counselors for the people of Enewetak. The
Enewetak Council expressed its desire that the laboratory
continue to function in the Enewetak community after the
return and resettlement of the atoll residents. It approved
the site of the Coast Guard LORAN Station as the ulti-
mate home of MPML.

With the approval of reorganization and redirection of
goals, the laboratory entered a new and productive phase.
Support and encouragement of basic studies continued
under AEC sfxjnsorship, while mission-oriented research
was being planned and implemented. The major AEC-
oriented projects of the 1975 to 1980 period were (1) a
study of the circulation of the Enewetak Lagoon, (2)
research on the d^ amies of groundwater resources of
Enewetak Atoll, and (3) studies of ciguatera fish poisoning
at Enewetak.

On Jan. 1, 1975, Philip Helfrich left the University of
Hawaii and was replaced as director of MPML by Stephen
V. Smith, who served in that capacity until 1977. During

A HISTORICAL PERSPECTIVE

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HELFRICH AND RAY

Smith's tenure the three research projects mentioned
above dominated the activities of the laboratory. A study
of the oceanography of Enewetak Lagoon was prompted
because — despite intensive studies of various facets of
Enewetak's geology, physiography, biota, ecosystem dy-
namics, radiation contamination, etc. — only cursory infor-
mation existed on the circulation patterns of the lagoon
(Chapter 5 of this volume). This comprehensive study
directed by Smith resulted in information on the physical
and chemical dynamics of the entire lagoon. The topic of
the second investigation was the dynamics of groundwater
resources of Enewetak, a study that developed information
vital to the returning Enewetak people who required
uncontaminated water for drinking and agriculture. This
investigation was directed by Robert W. Buddemeier
(Chapter 4 of this volume). Ciguatera fish poisoning, the
topic of the third study, had plagued the people of the
Marshall Islands for many years, waxing and waning in an
inexplicable manner. The return of the people and their
dependency on fish for sustenance placed a special
urgency on the results of this study that was directed by
John E. Randall (Chapter 7 of this volume).

During 1975, the AEC was reorganized, and the func-
tions pertinent to MPML were assigned to the newly
formed Energy Research and Development Administration
(ERDA). In turn, ERDA gave way to the U. S. DOE in
1977.

Resident managers were established at MPML on a
year-round basis in 1975, and these individuals became
integrated into the Enewetak community. This was an
important aspect of MPML's operations because these
scientists represented a benign, if not benevolent, element
among the numerous government-sp>onsored activities
related to the radiological survey, cleanup operations, and
various medical and agricultural programs. The individuals
who served as the resident laboratory managers were all
exemplary in their dedication, and there were numerous
examples of extraordinary service. From 1975 to 1977
the resident laboratory managers were Philip and Janet
Lamberson.

In June 1977, Ernst S. Reese assumed directorship of
MPML, replacing Smith. During Reese's tenure (1977 to
1979), the research on lagoon oceanography, groundwater
dynamics, ciguatera, and other aspects of atoll research
continued. Planning and implementation of the move to the
former Coast Guard LORAN Station took place. In addi-
tion to continuing to fully support the research mission of
MPML, the laboratory personnel cooperated in many ways
with the DNA. A highlight of this coojseration was the pro-
duction of an audio-slide presentation to acquaint the mili-
tary personnel of the DNA with the natural history of a
coral atoll and to describe the recreational opportunities
offered by the atoll environment. There was also a caution-
ary note about the dangers of the atoll environment rang-
ing from severe sunburn to the presence of sharks. The
audio-slide presentation contained an important message
about conservation of the atoll environment as well:
observe and enjoy but do not destroy.

Following the cleanup, support services were with-
drawn, and the laboratory was placed on a "stand alone"
status, having to provide for all of its own life support and
laboratory operations needs, with resupply from infrequent
supply ships and light aircraft. During this challenging
period, Reese was ably assisted by Victor R. Johnson and
Maridell Foster and by several capable resident laboratory
managers: Paul M. Allen, Michael V. DeGruy, and Gary
Long (1977 to 1979). In 1979, Patrick L. Colin and John
T. Harrison (1979 to 1983) took over the operation of the
laboratory. Throughout this period the laboratory contin-
ued to accommodate a few visiting scientists as transporta-
tion and logistics could be arranged.

In 1979, with the cleanup of Enewetak nearing comple-
tion and the return of the atoll's residents imminent, a
workshop was held at the Asilomar Conference Center,
Monterey, Calif., to consider the future role of the labora-
tory and its relationship to the other DOE scientific pro-
grams in the Marshall Islands. The DOE headquarters
sponsor at that time was the Division of Biomedical and
Environmental Research under the direction of Helen M.
McCammon. The fXDE policy enunciated at this time sig-
naled the ultimate phase down of the laboratory over the
following 2 to 3 years and the determination that signifi-
cant effort should he devoted to synthesizing the research
product of the laboratory's entire history into a publishable
work. The present volumes are the result. It was decided
also that, to the extent that the laboratory continued active
research programs during the phase down years, these
should not be confined to the marine environment. This
latter decision was reflected in yet another name change:
MPML became MPRL, the Mid-Pacific Research Labora-
tory. In 1980, soon after the Asilomar meeting, Helfrich
again assumed the directorship of MPRL.

For most of the time between 1977 and 1980, a large,
joint military force was at Enewetak — with a peak popula-
tion of about 1000 drawn from the Army, the Navy, the
Air Force, civilian government agencies, predominately
DOE and civilian contractors. Research at MPRL continued
through this period and in some ways the laboratory
thrived upon the ready availability of logistic support, espe-
cially frequent and dependable airlifts, and a generally har-
monious relationship with the joint cleanup command. In
fact, through the cleanup years, the resident manager of
the MPRL facility met daily with the Joint Task Group
Commander and his staff to discuss mutual interferences
and mutual supp)ort. Many interesting aspects of the
cleanup effort required an intimate knowledge of the atoll
system, and the laboratory was often called up)on for con-
sultation and advice. Selection of a suitable site for lagoon
disposal of debris, protection and exploitation of food
resources, and the preservation of scientifically valuable
artifacts were but a few examples. On one occasion a
major earth-moving eftort was planned for an island which
had unexpectedly become a nesting ground for a very
large flock of migratory birds. The laboratory's data base
facilitated an immediate assessment of the length of time
these birds would require protection, and it was possible to

A HISTORICAL PERSPECTIVE

reschedule the cleanup activities so as to have only a
minimal effect upon them.

The atoll rehabilitation program consisted of the re-
moval and disposal or isolation of debris and contaminated
materials, the construction of homes and community build-
ings and facilities, and the planting of more than 30,000
coconut, pandanus, and breadfruit trees. The cost was
over $100 million. In April 1980, a ceremony was held at
Enewctak, commemorating completion of the cleanup and
the return of 543 Enewetak people to their ancestral
home. A short time later, the last elements of the Joint
Task Group departed Enewetak, leaving the laboratory as
the only American presence in the community.

Over the next 3 years, major emphasis was placed
upon studies of a portion of the atoll ecosystem which had
until then been largely unexplored — the soft lagoon sub-
stratum. This research was directed by Patrick L. Colin.
Much of the fallout material which remained from the
nuclear tests had settled in the lagoon floor, and the
dynamics of this biotope were little understood. As a result

of this research, a fresh perspective was acquired. What
had formerly been considered to be a largely passive sys-
tem into which materials were sedimented from the water
column was revealed to be an area in which burrowing
organisms were continually reintroducing material into the
water column — a process which led to some revision of
the understanding of important biogeochemical processes.
Interest in these processes helped to stimulate interest, in
1981, in one more interdisciplinary initiative at Enewetak.

A significant improvement in understanding of the
deeper sediments of the lagoon required direct observation
and sampling, and these techniques required the use of a
research submersible. With the cooperation of the Hawaii
Undersea Research Laboratory, the research submersible
Makali'i was made available for a period in the summer of
1981 (Fig. 9). Other sponsors of the expedition were the
National Oceanographic and Atmospheric Agency (NOAA)
and the DOE. The DOE support included use of the
research vessel Liktanur. Fifteen scientists and seven sup-
port personnel participated in a program which included

Fig. 9 The research submersible Makali'i operated by the University of Hawaii shown on one of its 53 research dives in the
Enewetak Lagoon in the summer of 1981. [Photo courtesy of HURL Program, University of Hawaii.]

HELFRICH AND RAY

52 successful research dives between July 7 and Sept. 29,
1981. The results were presented in a special symposium
of the Western Society of Naturalists in Los Angeles in
December 1982 and were published in Bulletin of Marine
Science (Harrison, 1985).

AN ERA ENDS

Although the plans for an autonomous laboratory after
the 1980 departure of the cleanup forces were thought-
fully and thoroughly prepared and enthusiastically carried
out, and despite the welcome that MPRL had received
from the returning Enewetak community, its anticipated
position as a permanent fixture in that community was not
to be. At a time of constrained research dollars in the
DOE, and with support grants from all sources limited, the
cost of maintaining a resident staff and operating the
MPRL facility as a self-sustaining field station became
prohibitive. Support from the Division of Biomedical and
Environmental Research was terminated in 1982,
whereupon [X)E's Nevada Operations Office sought and
obtained funding for one more year through the DOE
Ofiice of Defense Programs. This additional year of fund-
ing permitted an orderly phase down of the laboratory
activities and the preservation of some of MPRL's unique
assets.

The reference collection which had been started during
Hiatt's early tenure had grown and had been well
preserved and cataloged. For several years this was
accomplished through a contract with the Bernice P.
Bishop Museum, under the able sufjervision of the late
Dennis M. Devaney. The collections were carefully pack-
aged and shipped to Hawaii to be placed in the temporary
custody of the Bishop Museum. Early in 1985, negotia-
tions were completed by the DOE with the Smithsonian's
National Museum of Natural History and with the Bishop
Museum for the permanent transfer of the reference collec-
tion to the latter institution. The MPRL's library and much
of the laboratory equipment were transferred to Hawaii
Institute of Marine Biology.

The remaining U. S. government activity at Enewetak
is now conducted on a campaign basis, usually supported
by the research vessel Liktanur. At this writing, however,
two [X)E contractor employees remain at the atoll, and
the field station remains intact and capable of limited sup)-
port. Philip Helfrich retains the title of Director of MPRL
and, with modest funding from DOE, entertains inquiries
from scientists who desire to explore the feasibility of con-

tinuing studies at the atoll. There is every indication that
the people of Enewetak would welcome such visits.

ACKNOWLEDGMENTS

The wisdom and foresight of H. Burr Steinbach and
Robert W. Hiatt and of those in the Office of Naval
Research and the AEC who spawned and nurtured the
idea of a research facility at Enewetak deserve special
note. Time has proven that the decisions to establish,
maintain, and support EMBL and its successors were wise
and fruitful commitments which resulted in important con-
tributions to our knowledge of atoll ecosystems and more
broadly to marine science. Assuredly, there are still many
unanswered questions, but just as surely new knowledge
will continue to be built up>on the foundation of about 250
published scientific papers which have resulted from
research conducted at Enewetak Atoll over the past 30
years. The writers of this chapter, who have been partners
in the administration and support of the laboratory for
almost half of that period, record their hop)e that new
ways will be found by interested scientists and their spon-
sors to continue, even on a limited scale, the exciting and
rewarding experience of research at this remote and iso-
lated atoll.

REFERENCES

Harrison, J T III, 1986, Recent Marine Studies at Enewetak
Atoll, Marshall Islands, Bull. Mar. Sci.. 38: 1-3.

Mines, N. O., 1962, Prouing Ground: An Account of the
Radiobiological Studies in the Pacific, 1946-1961, University
of Washington Press, Seattle.

Trust Territory of the Pacific Islands, Enewetak Atoll Master Plan,
1975, 3 volumes. Holmes and Narver, Inc., Anaheim, Califor-
nia.

U. S. Atomic Energy Commission, 1973, Enewetak Radiological
Surve^i. 3 volumes, Nevada Operations Office, Las Vegas,

Nvauo.

U. S. Defense Nuclear Agency, 1975, Environntental Impact
Statement: Cleanup, Rehabilitation, Resettlement of Ene-
wetak-Marshall Islands, 4 volumes, Washington, D.C.

U. S. Department of Energy, 1979, Mid-Pacific Marine Labora-
tory Contributions, 1 volume, Nevada Operations Office, Las
Vegas, NVa628-l.

— , 1982, Enewetak Radiological Support Project, Nevada Opera-
tions Office, Us Vegas, NVO-213.

U. S. Energy Research eind Development Administration, 1976,
Eniwetok Marine Biological Laboratori/ Contributions, 3
volumes, Nevada Operations Office, Las Vegas, NVO-628-1.

Chapter 2

History of the People of Enewetak Atoll

ROBERT C. KISTE

Director, Pacific Islands Studi/ Program
Uniuersify of Hawaii, Honolulu, Hawaii 96822

INTRODUCTION

The names of Enewetak and Bikini Atolls are linked in
history, and they are well-known around the world because
of their use as nuclear test sites by the United States.
Indeed, once the atolls became available as research sites,
a vast amount of research resulted; this volume is just one
of the results. Most of the research has been in the biologi-
cal and physical sciences, and the sheer volume of it has
tended to obscure a very important fact — Enewetak and
Bikini could be used for nuclear and other research pur-
poses only after their indigenous human populations had
been moved elsewhere. Much less is known about the
people than about the flora, fauna, and physical properties
of their atoll homelands. This chapter focuses upon the
people of Enewetak. It examines their history, the struc-
ture of their culture and society, the ways they have coped
with the colonial powers that governed the islands, and
their response to their resettlement on Ujilang Atoll. Some
mention is necessarily made of the Bikini community
because the histories of the two peoples are intertwined.

Data about the Enewetakese are mainly derived from
the research of four anthropxjlogists, all of whom worked
with the p>cople after their relocation. Jack A. Tobin was
the first. He served as Marshall Islands District Anthropolo-
gist between 1950 and 1957. He resided with the
Enewetakese on several occasions, and portions of this
work resulted in his doctoral dissertation (Tobin, 1968). In
1964, Leonard Mason and I spent several months on
Ujilang, and during the academic year 1972-73, I was
involved in a legal suit (to be discussed later) which
involved the Enewetakese and the U. S. Dejjartment of
Defense (Kiste, 1976). More recently, a younger anthrof)ol-
ogist, Laurence Carucci, spent 1977 and 1978 with the
Enewetakese, and he too produced a doctoral dissertation
(Carucci, 1980).

THE ANCIENT PAST

The research findings of prehistorians and linguists indi-
cate that the Marshalls and other islands of Micronesia
were settled by peoples who migrated from the general
area of island southeast Asia into the insular Pacific many
centuries ago (Bellwood, 1979). Indeed this particular
migration probably began about 5000 years ago. Reflect-
ing the ancient migration patterns out of island southeast
Asia, the Marshallese language belongs to the large Aus-
tronesian (also known as the Malayo-Polynesian) language
family which is spread from Madagascar, through
southeast Asia and across Micronesia, Polynesia, and many
regions of Melanesia. Exactly when the early migrants
arrived in the Marshalls is not known. The earliest archaeo-
logical date currently available for the Marshalls is from a
site at Majuro Atoll which was occupied at the time of
Christ. In all probability, future archaeological research will
push the date for the settlement of the Marshalls further
back in time.

No archaeological research has ever been conducted at
Enewetak Atoll, however, and it seems safe to assume that
remains of the past once deposited in its soil were
obliterated with the preparations for and by the nuclear
test program. The Enewetakese, however, have their own
version of the distant past. According to their oreil litera-
ture, they had always lived on Enewetak. In their own
words: "We were there from the beginning." At the same
time, their legends also recount how at least some of their
ancestors purportedly came from Bikini, Ujac, Wotto, and
other atolls also located in the northern Marsheills (Tobin,
1968).

Regardless of the time of the settlement of Enewetak,
two things are certain. Enewetak Atoll is isolated, and
once the ancestors of the current population were in place,
they had relatively little contact with other communities.
As a consequence, the language and culture of the
Enewetak people became differentiated from those of
other Marshallese, and the people did not identify them-
selves with the others. Indeed, they thought of themselves
as a people who were separate and unique, "the people of
Enewetak Atoll" as opposed to the islanders in the rest of
the Marshallese archipelago.

KISTE

The contact that the Enewetakese had with others, lit-
tle as it was, was not limited to the Marshalls. The oral
accounts associated with genealogies relate that some
Enewetak people, mainly males, occasionally sailed to the
south and west, contacting the ancient population of
Ujilang (included in the Marshalls) and on to the high vol-
canic and culturally and linguistically different island of
Ponape. Contact with Ponape was to continue well into
historic times and up until World War II.

Long before the advent of Europeans, the people of
Enewetak had developed a culture which represented a
good adaptation to the limited atoll environment which is
quite restrictive when compared to the high volcanic
islands of the Pacific. The people were skilled navigators
(an art which has been lost with the availability of travel
on the vessels of foreigners), and they were expert builders
of outrigger sailing canoes which were among the largest
in the entire Marshalls. (Well into the 1960s, the Enewetak
people were still constructing canoes that measured over
55 feet in length with masts that soared 30 feet above the
vessels' decks.)

In the relatively dry northern Marshalls and with the
poor soil of the northern atolls, terrestrial resources were
quite limited. Subsistence resources from the land were
limited to coconuts, pandanus, papaya, bananas, and
arrowroot. One or two breadfruit trees produced poorly.
None of these crops required much care, and the people
were very casual in their attitude about their maintenance.
A similar attitude was evidenced regarding domestic
animals. A few pigs and chickens were allowed to more or
less fend for themselves, and their flesh was mainly
reserved for holiday occasions.

Thus, in part, ecological necessity had caused the
Enewetak people to develop an economy which was
heavily reliant upon marine resources. They knew the
behavior and the monthly and annual movements of the
large inventory of marine fauna. The fish of the lagoon and
sea were caught, and expeditions were organized to collect
shellfish, capture lobsters and turtles, and gather turtle
eggs. In addition, several species of birds were also cap-
tured as food resources.

Shortly after the beginning of the German colonial era,
old patterns were altered and the people became involved
in the copra trade. Coconuts were converted to copra for
cash and/or trade goods. Rice, flour, sugar, coffee, tea,
canned meats and fish were eventually added to the diet.

Several other features of the people's lifestyle deserve
mention. Like most atoll dwellers, the people located their
residences on the largest islands of their atoll. In the case
of Enewetak Atoll, only the two largest islands were inhab-
ited: Enewetak Island in the southeastern quadrant of the
atoll and Enjebi Island on the atoll's northern rim.

Although permanent residences were located on
Enewetak and Enjebi Islands, the people were quite mobile
within the atoll. Fishing and collecting activities penetrated
every niche of the environment. Regular expeditions were
made to all islands in the atoll to make copra and to col-
lect food resources. Clearing brush and planting were done

during these visits. Except for holiday seasons, it was not
unusual for half of the population to be away from the two
main islands as the p>eople dispersed in pursuit of a liveli-
hood and for pleasure. Such expeditions broke the monot-
ony of life on a small island and provided relief from one's
fellows.

SOCIAL ORGANIZATION

Although the people had a collective identity as
Enewetakese when juxtaposed to other Marshallese, they
were divided internally into two separate communities that
resided on Enewetak and Enjebi Islands. Community is
defined as "the maximum group of persons who normally
reside together in face-to-face association" (Murdock,
1949). Members of the two communities intermarried and
cooperated in a variety of activities. Each functioned, how-
ever, as a separate social and pwlitical unit, and its
members had separate identities. The people of the
Enewetak community called and thought of themselves as
riEnewetak (the people of Enewetak Island) and those of
the Enjebi community were riEnjebi, (the people of Enjebi
Island).

The traditional settlement pattern of both communities
was dispersed. Residences were located on separate land
parcels known as wato and were scattered along both sides
of a sand and coral roadway which ran parallel to the
length of the lagoon beach. In most cases, a uxjto was a
strip of land which cut across the width of an island from
lagoon beach to oceanside reef. They varied in size from
about 1 to 5 acres. Each wato had a name, and the people
who lived on Kabnene wato on Enewetak Island were
sometimes referred to as riKabnene.

The two communities had the same political structure.
Each was headed by a hereditary chief known as iroij (Fig.
1). The chiefs directed the affairs of their respective com-
munities, arbitrated disputes, and consulted one another
with regard to concerns of the entire atoll and the total
population's relations with outsiders (Fig. 2). In contrast to
other Marshallese communities, which are organized
around matrilineal principles, succession to the chieftain-
ship was patrilineal, i.e., a man was succeeded by his eld-
est son; the eldest son was succeeded by his younger
brothers in the order of their birth; and when the last of
them died, the eldest son of the eldest son succeeded.

Like other Marshallese, the people of Enewetak Atoll
were divided among several matriclans. The clans were
named, and every individual automatically became a
member of his or her mother's clan at birth. Clan member-
ship could not be altered. The clans were vehicles for the
provision of hospitality. One was obligated to protect fel-
low clansmen and to provide them with food and shelter
(Fig. 3).

The clans were exogamous, i.e., members were
required to marry outside of their clan. Members treated
their clansmen as if they were parents or siblings, and sex
within the clan was tantamount to incest. The preferred
marriage partner was a real or classificatory cross-cousin

HISTORY OF THE PEOPLE

"19

,.*«W

J^ ■■■*?' '^^

Fig. 1 Iroij (Chief) Joannes Peter and his wife Bela. Ujilang
Atoll, Nov. 17, 1976. [Photo by Janet Lamberson.]

Fig. 2 Luther, an Enewetak elder and a repository of tradi-
tional cultural wisdom. Ujilang Atoll, Nov. 17, 1976. [Photo
by Jcinet Lamberson.]

(father's sister's daughter or mother's brother's daughter),
and a very high percentage of marital unions were of the
preferred type.

Ideally, postmarital residence was patrilocal. A male
took his bride to live on his father's land. Sometimes
newlyweds lived with the man's parents, but the couple
usually built a separate dwelling nearby. Quite commonly,
a man and his married sons occupied adjacent dwellings
but shared a common cooking house which was a separate
structure. Thus, a patrilocal extended family was the most
common family group located on a given wato.

Another facet of Enewetak Atoll culture that differed
from that of the rest of the Marshalls was the system of
land tenure and inheritance. In contrast to the rest of the
Marshalls where matrilineages (subunits within the matri-
clans) constitute landholding corporations, the land tenure
system at Enev,(etak Atoll was bilateral. In most cases, a

married couple divided the land they had each inherited
among their children, and a child usually received some
land from both his or her father and mother. As the paren-
tal generation died and as members of the next generation
married and produced children, the process was repeated
with parents allocating land among their offspring (Fig. 4).

The people had an almost mystical attachment to their
land, and their ties to it were deep. They could trace the
history of their holdings back about a half-dozen genera-
tions. As indicated previously, an individual's identity was,
at least in part, defined by one's urate and one's island of
residence.

A final important social institution was an import. The
people of Enewetak Atoll were the very last in the
Marshalls to experience missionization because of their iso-
lation and distance from the wetter, more richly endowed
southern atolls where colonial powers always had their

KISTE

Fig. 3 Aruo, a canoe builder and sailor, was lost at sea at
Enewetalc Atoll in 1983. [Photo was taken at Ujiland Atoll in
1977 by Janet Lamberson.]

headquarters. Not until 1927 did a Protestant missionary
arrive to bring fundamental change to the people's world
view. The first missionary was an islander from Mokil Atoll
in the eastern Carolines, and he was followed by another
missionary from Kosrae. The outsiders did not remain
long, however, because within a few years a member of
the ri£neaieta/c community was trained to lead the spiritual
life of the people.

The church took firm root. As in most places
throughout the Pacific, the pjcople fully embraced Chris-
tianity. Its teachings were mixed with traditional beliefs
about ancestral and nature spirits and other notions about
the supernatural, and the result was a hybrid that had
become an integral part of the local culture and society.
Work and play were tabu on Sundays. Other church ser-
vices were held during the week. Christmas and Easter
were the major holidays of the calendar year.

COLONIAL HISTORY

The Spanish explorer Alvaro de Saavedra is given
credit for the European discovery of Enewetak Atoll in
1529. After his initial contact, like many other islands
and atolls in the Marshalls and Carolines, Enewetak was
not visited again by Europeans for many decades. The
next known sighting of the atoll occurred in 1792, and 2
years later another European vessel called. In 1798,
Enewetak Atoll was mapped by a Captain Fearn in
command of the Hunter (Tobin, 1968). Although contact
with the outside world surely has made some impression
on the people, it seems somewhat odd that no accounts of
early Europ>ean visitors were found in the oral history of
the people.

In 1898, shortly after the Germans had declared the
Marshalls to be a Protectorate, a German trading company
contracted the Enewetakese to extend their plantings of
coconut palms for the copra trade. Some of the people
traveled to Ujilang Atoll to work on the copra plantation
there under a German supervisor. German rule was brief,
however, and no German or other outsider actually took
up residence on Enewetak during German times. In fact,
the people were still adjusting to the European interlopers
when Japanese colonial rule replaced that of the Germans
in 1914 (Kiste, 1977).

Because they are much closer to Ponap)e Island in the
eastern Carolines than the old colonial headquarters at
Jaluit Atoll in the southern Marshalls, Enewetak and
Ujilang Atolls were administered and visited by Japanese
vessels from Ponape during Japanese rule. Consequently,
the Enewetakese were separated even more from other
Marshallese. It was also during Japanese times that the
people lost some of their autonomy and lessened their con-
trol over their land. Japan began its rule with a show of
force by sending naval vessels to confirm Japan's author-
ity. In the early 1920s, a Japanese trader established him-
self on the atoll. He falsely claimed that the colonial
government had granted him p)ermission to acquire land
and develop coconut groves. He also claimed that the peo-
ple were required to assist him with the venture. Initially
the Enewetakese did not resist and worked for modest
rewards in trade goods, but as they became more familiar
with the Japanese, they realized they had been duped, and
the two chiefs filed a complaint with officials. The issue
was not resolved before the Japanese military began to for-
tify the atoll in the late 1930s as part of the preparations
that led to World War II.

The war years brought tragedy. First, the Japanese
constructed an airstrip on Enjebi Island and evicted the
riEnjebi to a small corner at the eastern end of their island.
The American invasion in 1944 devastated and practically
denuded both the Enjebi and Enewetak Islands. Ten per-
cent of the local population was killed. At the end, both
communities were moved to two small islands in the east
side of the atoll. The Americans constructed a large mili-
tary base on Enewetak Island, and the people acquired
their third colonial master. When the Americans asked

HISTORY OF THE PEOPLE

Fig. 4 The Enewetak children represent the promise for the future. UJilang Atoll, Nov. 17, 1976. [Photo by Janet Lamberson.]

them to abandon their homeland, the Enewetakese
correctly concluded that they had no real alternative, so
they offered no resistance (Kiste, 1977).

THE UJILANG RESETTLEMENT

Ujilang is 124 miles southwest of Enewetak. It had
been inhabited by a Marshallese population, but in the late
1800s a typhoon decimated the atoll and killed all but a
handful of its people, most of whom were moved to the
southern Marshalls. Ujilang was then developed as a com-
mercial copra plantation during the German eind Japanese
eras, and as noted, some of the p>eople of Enewetak Atoll
had experiences there as laborers during German times.
Ujilang was abandoned during World War II, and thus it
was available to receive a population.

American authorities initially thought of Ujilang as a
site for the relocation of the Bikinians. They were the first
to be moved to make way for the nuclear tests. Their first
relocation occurred in March 1946 when they were moved
to nearby Rongerik Atoll. It had never had a permanent
population of any size, and the reason soon beceime

apparent. Rongerik's resources, greatly overestimated by
American planners, were inadequate to support the com-
munity. After considerable delay and many complications,
the Americans decided to move the Bikinians to Ujileing,
and in November 1947, an advance party of Bikini men
and navy Sea bees arrived to construct a village. In less
than 2 weeks, however, officieils in Washington, D. C.
announced plans to use Eneweteik as a second test site,
necessitating a relocation of its inhabitants. They were
moved to Ujilang on Dec. 21. The Bikinians were eventu-
ally resettled on small Kili Islemd in the southern Meu'sheills
where they have never made a satisfactory adjustment
(Kiste, 1974).

Ujilang has only one sizeable island, 2tnd both the
riEnewetak and riEnjebi communities were resettled there.
The islemd was evenly divided by an Americiin naval offi-
cer who ckllotted one half to each community. A rather
compact village was constructed in the middle of the
island, with the Enewetak and Enjebi people residing on
their respective sides of the dividing line. No longer
separated by Enewetak 's large lagoon emd with the more
compact settlement pattern, the two groups became a sin-
gle community while retaining their dual political structure.

KISTE

The years on Ujilang were quite difficult. The atoll is
much smaller than Enewetak and has correspondingly
fewer resources. Enewetak has 39 islands with a total land
area of 2.75 square miles; its large lagoon covers 387.99
square miles. In contrast, Ujilang has 21 islands which col-
lectively constitute only 0.67 square miles (Holmes and
Narver, 1975; Tobin, 1968).

Compounding the problem of living on a smaller atoll
with a greatly reduced resource base, the people, like
other Micronesians, have rapidly increased in numbers.
The total population at the time of relocation was only
141. By the early 1950s, the number had increased to
about 170. By 1977, the population had reached 400
(Kiste, 1977). A census taken in 1978 reported 540
(Carucci, 1980), and today the number is probably in the
vicinity of 600, a four-fold increase since relocation.

Population pressures on Ujilang's resources obviously
increased during the people's years on the atoll, and on
numerous occasions, food supplies from the land were
depleted. Coconuts that might have been converted into
copra were needed for sustenance, and as a consequence,
the people had little cash to purchase imports. The situa-
tion was exacerbated because Ujilang is distant from the
government center at Majuro, and ships carrying food and
other supplies frequently failed to service the atoll. As a
result, the people suffered considerable physical depriva-
tion. For those who knew it well, memories of life at
Enewetak brought despair, and younger people became
convinced that they had been deprived of their true home
where want was unknown. The desire to return to
Enewetak increased with each passing year (Kiste, 1977).

In spite of the adversities suffered and the periods of
discouragement, the people always maintained a great
sense of pride in themselves and a determination to control
as much of their destiny as possible. EXiring the initial
years of U. S. rule, the jseople sized up the Americans and
attempted to determine the best ways of dealing with
them. Until the mid-1960s, they tried to get help by mak-
ing complaints and fjetitions to the administration. Welfare
measures were occasionally implemented, but more often
than not, the people's pleas went unheeded. During this
period, the traditional political structure remained intact.
The chiefs functioned in their usual roles, and as many
traditional leaders elsewhere, they resisted American
efforts to introduce Western political forms — in this
instance, a council form of government headed by an
elected magistrate. By the early 1960s, however, some
change was observable. The two chiefs were by then older
men. Some contemporary issues required that the
decision-making processes be opened to include younger
men who had attended American schools and/or had been
employed by the administration. Meetings of adult males
were occasionally held, and some decisions about commu-
nity affairs were decided by a majority vote.

In 1967, exceptionaDy poor conditions on Ujilang and
a realization that previous pleas to the administration had
largely been ineffective prompted the people to take a
much more aggressive stance. After an absence of 6

months, a field trip vessel called. Much to the surprise of
the official in charge, the people boarded the ship and
announced their intention to abandon the atoll. A poten-
tially dangerous voyage on an overloaded ship was
avoided when the officials volunteered to remain on the
atoll and "suffer from starvation" until the administration
responded to the situation. The display of assertiveness
produced results. Substantial amounts of food and other
supplies were soon delivered, and the District Administra-
tor of the Marshalls came to hear the people's grievances.

The sit-in aboard ship and another threat to abandon
Ujilang a year later had the greatest support from younger
adults. The sit-in also seems to have been linked to a
major transformation in the community's political structure.
Sometime during late 1967, the two chiefs had yielded to
younger men. A magistrate and a council of 12 were
elected. Reflecting the traditional division of the popula-
tion, the riEnjebi and the riEnewetak each elected six coun-
cilmen. The magistrate became the head of the entire com-
munity; the council became its legislative body. The chiefs,
however, continued to function importantly as advisers and
men of substantial influence (Kiste, 1977).

In 1968, the people evidenced considerable sophistica-
tion about the larger world when they petitioned the
United Nations for assistance in returning to Enewetak. In
August, it was learned that Bikini was judged to be safe
from radiation and that it could be returned to its people.'
The news caused great resentment among the riEnjebi and
riEnewetak, and they strongly protested their continued
alienation from home. The protest produced results. In
1970, in an effort to satisfy the people, the United States
Congress authorized a payment of $1,020,000 to the peo-
ple of Enewetak. Other payments were to follow in later
years.

The initial attempt to placate the people was not suc-
cessful. In late 1971, they announced their intention to
return home before the end of the following year. Depart-
ment of Defense (DOD) officials contended, however, that
it was necessary for Enewetak to remain under DOD's con-
trol. This was rejected, and by early 1972, the people
obtained legal counsel from the recently created Microne-
sian Legal Services Corporation (MLSC). The people then
informed officials that they would institute legal action if
Enewetak was not returned to them. On April 18, 1972,
the long-awaited day arrived; it was announced that the
U. S. would surrender Enewetak by the end of 1973 after
certain "unspecified activities" had been completed there.

The p)eople had won a major victory, but it soon
became apparent that the "unsp>ecified activities" were a
threat to their future well-being. The activities were part of

*ln 1968-69 a cleanup was conducted at Bikini Atoll, and a
residential complex was established About 140 Bikini people
returned to Bikini in the early 1970s, but by 1978 it became
apparent that the radiation content of foods grown at Bikini made
permanent residence there inadvisable. The Bikinians were again
removed from their atoll and, at this writing, have not yet
returned.

HISTORY OF THE PEOPLE

the Pacific Cratering Experiments (PACE) project and were
sponsored by the DOD and related agencies. PACE had
commenced with small explosions and was projected to
culminate in several multiple ton detonations of high explo-
sives and one final 500-ton blast. It was hoped that this
series of experiments would help to provide a better
understanding of many of the effects of the tests of the
1950s. The f)eople of Enewetak, represented by their
MLSC lawyers, invoked the provisions of the National
Environmental Policy Act, and they filed suit in the Federal
District Court in Honolulu in September. At Ujilang, PACE
scientists explained their project, claiming that it would
cause no long-term damages. The people listened politely
and responded with a brief but very firm statement. In
essence, they stated: "PACE is evil, and we will do what-
ever we can to prevent it." The magistrate gave an elo-
quent speech which reflected the people's values and
feelings.

I do not know if you have made an attempt to compare
your sense of values, you who live in America or else-
where, with ours. You live with gold and money and we
have to depend on land and whatever life we can find on
land and in the water. Without these, we are nothing. We
do not have to explain further that Enewetak, with what-
ever land resources and whatever marine resources it has,
is our homeland, and seeing that you understand this, we
do not know why you continue to insist to do these things
on Enewetak, when for us there is really nothing else to
look forward to. For this reason we must continue to ask
that you refrain from proceeding with this program. PACE
is no good . . . Enewetak has undergone severe damage.
There are islands that are missing. There is a considerable
amount of land that has been destroyed. The question then
comes: Has not Enewetak done enough for your testing?
We do not know who you will take this message
to — perhaps you will take it to Washington or the Depart-
ment of Defense — but, the point still remains that we feel

Fig. 5 Official ceremony returning Enewetak Atoll to its former inhabitants. Enewetak Atoll, Sept. 16, 1976. From left to right,
seated at the table, are Oscar DeBurum, then District Administrator of the Marshall Islands; Binton Abraham, Iroij (Chief) of the
liEnewetak, now deceased; Thomas Lacy, Brigadier General, U. S. Air Force, then Field Commander, Defense Nuclear Agency;
Peter Tali Coleman, then Deputy High Commissioner of the Trust Territory of the Pacific and later Governor of American Samoa;
Joannes Peter, Iroij (Chief) of the riEnewetak; Hcrtes John, magistrate of Ujilang Atoll. [Photo by Janet Lamberson.]

KISTE

that Enewetak has done enough. We have sacrificed
enough and PACE should not be continued because it only
means further destruction of our homeland. [Office of the
Judge Advocate Pacific Air Forces, 1973.]

The legal suit was never brought to trial as the DOD
cancelled the PACE project soon after the public hearings
(Kiste, 1976).

That the magistrate and not the chiefs spoke for the
people reflected the changes that had occurred in their po-
litical organization. By the time of the PACE affair, further
change had occurred because the process of electing coun-
cilmen had been altered. In elections subsequent to 1967,
the 12 councilmen were elected from the population at
large and not half from the Enewetak and half from the
Enjebi sides of the community. It appeared that the old
division between the two sides had lost some of its mean-
ing.

RETURN TO ENEWETAK

After the PACE affair, the people exjjerienced some
reversals. Radiological surveys revealed that some islands
of Enewetak Atoll are more heavily contaminated by
radioactive debris than previously thought, and they can-
not be inhabited for decades to come. In 1976, after
extensive radiological surveys, it was determined that
Enewetak Island and several others on the atoll's eastern
rim could be partially restored with reasonable safety. The
U. S. Congress provided funds for their cleanup and reha-
bilitation. The full-scale cleanup effort began in late 1977.
The Enewetakese were consulted in the planning and some
were employed to help with the work. The cleanup of
Enewetak Atoll, the construction of dwellings and commu-
nity buildings, and extensive replanting was completed in
1979, and the atoll was officially returned to the people in
April 1980 (Figs. 5 and 6). The event was celebrated by
virtually the entire papulation with 542 people attending.

Fig. 6 Iroij Joannes Peter signing documents returning Enewetak Atoll to the liEnetoetak and riEngebt. Enewetak Atoll,
September 16, 1976. [Photo by Janet Lamberson.]

HISTORY OF THE PEOPLE

OTHER ISSUES

Although the Enewetak case is unique, the people
share some historical trends with other Micronesians. Like
other islanders, the people of Enewetak have had to
become familiar with the representatives of the successive
colonial administrations. The Enewetakese had to learn the
customs of the new foreigners and had to develop ways to
cope with them.

The initial years of American rule followed on the
footsteps of World War II, and it was a time when
memories were still fresh of the destructive powers that
the U. S. had unleashed during its crushing defeat of
Japan in the Pacific. Understandably, Micronesians were
cautious and even timid in their dealings with Americans.

With the passing of time, Micronesians everywhere
grew bolder and became more skilled as they managed
their relations with Americans. Encouraged by this relation-
ship, Micronesians have modified their traditional institu-
tions and have adopted more democratic p>olitical
structures. In recent years, and very much like the people
of Enewetak, they have become more assertive as they
have negotiated for what they believe are their own best
interests. Inspired by the general wave of decolonization in
the Pacific, and as the end of the U. S. trusteeship draws
near, Micronesians have been struggling to take control of
their own lands and destinies. Self-government is coming to
the U. S. territory, and it seems unlikely that situations

such as those which occurred at Enewetak or Bikini will
ever occur again.

REFERENCES

Bellwood, P., 1979, Man's Conquest of the Pacific, Oxford

University Press, New York
Carucci, L., 1980, The Renewal of Life: A Ritual Encounter in the

Marshall Islands, unpublished Ph.D. dissertation, University of

Chicago
Kiste, R. C, 1974, The Bikinians A Studv in Forced Migration,

Benjamin/Cummings Publishing Company, Menio Park,

California
— , 1976, The Peoples of Enewetak vs. the U. S Department

of Defense, Ethics and Anthropology, M. A. Rynkiewich and

J. P. Spradley (Eds.), John Wiley and Sons, New York.
— , 1977, The People of Enewetak: Past and Present,

Micronesian Perspectiue. 1(2): 18-23.
Murdock, G. P., 1949, Scxial Structure, Macmillan Company,

New York.
Office of the Judge Advocate Pacific Air Forces, 1973, Transcript

of Testimoniyi Enuironmental Hearings "Project PACE," Hono-
lulu, p. 79.
Tobin, J. A., 1968, The Resettlement of the Enewetak People,

unpublished Ph.D. dissertation. University of California,

Berkeley, pp 18, 22, 57.
Trust Territory of the Pacific Islands, Enewetak Atoll Master

Plan, 1975, 3 volumes. Holmes and Narver, Inc., Anaheim,

California

Chapter 3

Ph\;siograph\; of Eneivetak Atoll

PATRICK L. COLIN

Motupore Island Research Department
(Jniuersity of Papua New Guinea
Port Moresby/. Papua New Guinea

LOCATION AND SIZE

Coral atolls have been variously defined and, without
considering unusual cases, can be described as more or
less continuous reef (largely corals and other calcium car-
bonate producing organisms), which surrounds a deeper
lagoon and drops steeply to oceanic depths on the seaward
margin. All islands are typically low, derived from reef rub-
ble and sand. Enewetak Atoll conforms to all aspects of
this description and in many respects is a "textbook" atoll.
It has a large elliptical lagoon, approximately 41 islands on
its rim, a few passages between the lagoon and ocean, and
narrow shelves dropping steeply into deep water on all
sides. The subsurface geology of Enewetak and Bikini have
been extensively examined, and these results are reported
in the U. S. Geological Survey Professional Papers 260
series.

Enewetak Atoll is located in the northwestern Marshall
Islands with its center at approximately 11°30'N;
162n5'E (Fig. 1). It is 220 km from the nearest land,
Ujelang Atoll to the southwest; 310 km from Bikini Atoll
to the east; and about 410 to 460 km from other atolls
(Ujae, Wotho, Ailinginae, Rongelap) to the southeast to
east. To the north occur Wake Island, about 1000 km
northeast, and Marcus (Tora Shima) Island, about 1600
km northwest. To the west are the Marianas, the nearest
being about 1700 km. All islands of the Marshall Islands
are low, most being coral atolls. The high islands nearest
to Enewetak are Ponape, to the southwest, and Kusaie, to
the south, both about 580 km distant.

Enewetak is a relatively large atoll, somewhat elliptical
in shape, about 33 by 41 km in size, with the islands, reef
flat, and lagoon covering about 1000 km . It is the third
largest atoll in the Marshall Islands, exceeded by Kwajalein
(the largest atoll in the world) and Rongelap. By world-
wide standards, it is not exceptionally large. The majority
of the area of Enewetak is the lagoon, with the reef flat
and the islands covering progressively less area. Table 1

provides information on the area covered by various
environments at Enewetak.

WEATHER AND CLIMATE

Weather at Enewetak is dominated by the surrounding
marine conditions. Since all islands are low and of small
area, they do not alter weather conditions by their pres-
ence. The atoll is semiarid, with rainfall averaging only
about 1700 mm per year, and has a distinct wet-dry
annual cycle. Air temperatures are relatively high and very
stable, with a mean annual temperature of about 28°C.
Solar radiation is intense, and humidity is consistently
high.

At almost 12°N, Enewetak is within the trade wind
belt with nearly consistent easterly winds. The atoll is sub-
ject to tropical storms and typhoons at irregular intervals
which greatly affect the marine and terrestrial environ-
ments. The meteorology of Enewetak is discussed in
Chapter 6 of this volume.

ENVIRONMENTS OF ENEWETAK
The Lagoon

The lagoon is the largest component of the atoll. It is
relatively deep by atoll standards, averaging about 54 m,
with a reported maximum of 71 m. The lagoon bottom
generally slopes from the lagoon rim toward the center. At
a distance of 2 to 4 km from the rim, the lagoon bottom is
essentially flat at a depth of about 45 m. Even the outer-
most 2 to 4 km of the lagoon has generally low slope gra-
dients on its bottom because of the horizontal distance
required to reach 45 m depth. The only areas with signifi-
cant slopes, except along the flanks of patch reefs and
coral pinnacles, on the soft bottom of the lagoon occur
shallower than 25 m. Below that depth, except for small-
scale undulations, there is little variation in the soft bottom
from the flat and horizontal. The area above 25 m depth
is also affected by wave action and currents which can
affect sediment distribution.

Most of the lagoon bottom is relatively inaccessible to
human observers. The depths are below those practical for
sustained diving operations and, generally, must be
observed or sampled remotely. The area of the lagoon bo'

COLIN

s
<

o291 3

PHYSIOGRAPHY

TABLE 1

Areas of Environments at Enewetak Atoll
(in kilometers)

Total atoll (land and shallow water

1022

less than 100 m deep)

Total land

7.125

Total marine environment

1015

Total lagoon depth in meters

938

Oto 10

10 to 20

20 to 30

30 to 40

103

40 to 50

253

50 to 60

310

Over 60

Outer reef slope

est. 13

Reef flat (less than 1 m at low tide)

torn visible in aerial photographs is limited to depths of 15
to 20 m and is usually located only on the rim of the
lagoon. The only structures which are visible from the sur-
face in the central lagoon are coral pinnacles which reach
within less than 15 to 20 m of the surface.

There arc two major channels between the lagoon and
ocean (Figs. 2 and 3). The first is the "deep" channel,
between Medren and Japtan, which is nearly 60 m deep in
places but is relatively narrow. It averages only about 1.4
km in width between Japtan and Medren, but the deepest
portion (below 40 m depth) is only about 600 m wide.

During tidal changes, swift currents flow in and out of this
channel It is exposed to the easterly swell from the ocean
and allows such swell to enter the lagoon in its vicinity.
The swell, combined with wind-produced chop due to the
open fetch of the channel and currents flowing out of the
lagoon (counter to the wind direction), often produces
extremely rough conditions in the channel.

The deep channel splits into two branches just west of
Jedrol Island leaving an area of shallow reef in between
with minimum water depths of about 6 m (Fig. 2). This
wedge-shaped reef gradually deepens both to the west and
north until it essentially merges with the lagoon bottom.
Near its easternmost extremity, a ferro-cement barge — the
"Concrete No. 9," locally called the "cement ship" — ran
aground, resulting in a distinctive marker of this site. The
bottom slopes away at about a 45° angle into the
branches of the deep channel which begin to flatten out at
about 40 m depth.

The second major passage, the "wide" channel, is
located at the south end of the atoll between Enewetak
and Ikuren. It is no more than 15 to 18 m deep but
stretches 10 km between the islands (Fig. 3). Since it is
considerably shallower than the deep lagoon bottom, it
resembles a sill. The currents in its vicinity are essentially
unidirectional, out of the lagoon (Atkinson et al., 1981),
but their speed is determined by the tide. Although the
wide passage does not directly face the ocean swells, the
swells are refracted somewhat around the southern end of
Enewetak Island and enter the lagoon through this open-
ing. This, combined with waves from the lagoon and the

Fig. 2 Aerial view of the deep, narrow channel entrtuice to the lagoon between Medren and
Japtan Islands on the eastern, windward side of the atoll. [Photo by P. L. CoUn.]

COLIN

Fig. 3 Aerial view of the wide, south channel passage to the lagoon looking from Enewetak
Island (lower right) to Ikuren Island (upper left). The shallow bottom of the sill at the passage
is visible. [Photo by P. L. CoHn.]

shoaling nature of the bottom at the wide passage,
produces rough conditions with standing waves and steep
waves in the western half of the wide passage.

A series of shallow open passes with fingers of emer-
gent to near emergent reef intersjjersed between them is
called the "southwest passage," an additional passage
between the lagoon and ocean. These openings cover
about 6.7 km of the atoll margin from the island of Biken
to the beginning of unbroken shallow reef to the southeast.
The sand-bottomed passes appear deeper to the south — as
much as 8 m deep in places. While significant, the
southwest passage is p>erhaps an order of magnitude less
important in lagoon-ocean water transport than the deep
and wide channels (Chapter 5 of this volume).

The reef flat is also a major source of water movement
into or out of the lagoon. The amount of such transport is
dependent on the height of the tide and the wind and
waves which influence the wave pumping of water from
ocean to lagoon. Where islands disrupt the free flow of
water across the reef flat into the lagoon, water flow is
channeled into narrow, deeper areas where current speed
can be relatively high. These channels are variously termed
"rips" or "gutters" and can also occur on intraisland reef
flats where there are areas of higher current flow.

The biological communities and environments of the
lagoon are discussed in Chapters 7 and 8 of this volume.
They are quite variable from place to place, varying from
sediment-bottomed areas devoid of hard substratum to
well-developed coral reefs. The diversity of plants and
animals is as high in the lagoon as it is in other areas of
the marine environment.

Emery et al. (1954) reported over 2000 "coral knolls"
in the lagoon with some suggestion that they "belong to 2
distinct size categories; nearly all the large coral knolls
have a diameter in excess of 1 mi whereas nearly all the
rest are smaller than Vi mi, and intermediate sizes are not
common." Most of these do not reach sufficiently close to
the surface to be visible and can be detected only by echo
sounding. Emery et al. (1954) distinguished between the
term "coral pinnacle" and "coral knoll," preferring the
latter term, but did not clarify how the reef structures of
the lagoon margin were considered. In essence an inter-
grading series of reef structures exists within the lagoon.
Although distinct types — such as coral knolls (broad, rela-
tively low structures), coral pinnacles (high relief relative to
diameter), and patch reefs (small structures, often in sheil-
low water) — can be identified, intermediates are common.
Those reef structures that are present on the bottom and
visible from the air are generally, in this treatment, con-
sidered to be "patch reefs."

The Reef Flat

The shallow reef flat, much of which is emergent at
low tides, around the rim of the atoll has been the most
intensively examined marine environment. It consists of
areas of rock pavement with seaward algal ridge structures
and lagoonward rubbly bottom. The reef flat varies consid-
erably in different areas of the atoll, particularly between
the windward and leeward sides but also over relatively
short distances on the windward shore. Very little of the
algal ridge, normally produced by coralline algae, is "live"
at Enewetak. Instead of the healthy pink corraline areas

PHYSKDGRAPHY

which have the characteristics of typical algal ridge struc-
tures, they are covered with fleshy algae. Indications are
that these areas were live algal ridges sometime within the
relatively recent past, but whether man has played a role
in their demise is uncertain. There is one small area of live
algal ridge still present at Enewetak, near the island of
Ananij, which occurs at the easternmost extension of the
reef flat. This and the ecology of the reef flat are discussed
in subsequent chapters.

The Seaward Slope

The seaward slope from the reef flat to the dropoffs to
depths over hundreds of meters is narrow all around
Enewetak. The edge of the seaward slop>e is marked by
"spur and groove," alternating reef and rubble fingers
projecting seaward where the waves break. On leeward
reefs, there are no distinct spur and groove formations but
a deeper series of promentories and reentrants in the
upper 15 m. On windward reefs, a rock bottom then
slopes away gradually to a break point at the 18 to 30 m
depth where the bottom begins to slope much more
steeply. Oceanic depths are quickly reached.

The width of the seaward shelf varies around the atoll.
It is widest off Enewetak Island, being about 400 m wide.
Other areas of the windward reefs are narrower so that it
is only 100 to 200 m wide on the northeastern reefs
between Lojwa and Enjebi. On leeward shores the shelf is
very narrow, only a few tens of meters wide. It is literally
possible to stand on the reef flat and throw a stone into
depths of 100 fathoms.

The Islands

There arc approximately 40 islands at Enewetak,
excluding a few small sand islands remaining above water
at high tide. Two islands (Elugelab and Lidilbut, not shown
in Fig. 1) were vaporized by nuclear testing, and three oth-
ers were so severely altered that only small remnants
remain (located in the northwest part of the atoll). The
vegetation of most islands at Enewetak has been progres-
sively and increasingly altered compared to the nondis-
turbed state. The alterations occurred initially by the estab-
lishment of coconut groves, later by wartime construction
and damage, and finally by nuclear testing activities and
subsequent military activity. Extreme alteration occurred
again during the Enewetak cleanup with the aim of re-
establishing the coconut groves. The only islands which
have essentially undisturbed vegetation are the five south-
western islands and Biken (see Chapter 11 of this volume
and Chapter 3 of Volume II for further details).

The islands can be grouped into several reasonably
natural units defined by significant gaps between units and
identified by direction location. Often these units are identi-
fied by compass location and are defined here.

The "southwest islands" of Enewetak are the five
islands from the southerly Kidrenen through Ikuren. They
are separated by both distance and intervening passes
from isolate Biken, which will be termed the "western

island," and from Enewetak Island. The islands of
Enewetak, Bokandretok, and Medren are called "south-
eastern islands"; they share a common reef flat and are
separated from all others by passes. The ten "central
islands" are those of the windward side from Japtan
through Runit, including Jedrol. The "northern islands" are
those 15 to 16 islands from Bijire or Billae through the
northern Boken. They are separated from the central
islands by several kilometers of op)en reef flat. The last
group, the "northwest islands," from Bokoluo to Luoj, are
separated from the northern islands by the large MIKE and
KOA craters and consist of four islands and one sand bar.

The islands consist largely of coral sand, rubble, and
boulders with areas of exposed beach rock and reef flat
pavement. In certain areas, large quantities of cement
debris are incorporated among coral boulders and rubble.
All the islands are low, the highest elevation being approxi-
mately 4 m on Enewetak Island. Beaches occur on many
lagoon shores, the most extensive and continuous today
found on Medren. Enewetak Island in pretesting days pos-
sessed an apparently continuous beach, but the lagoon
shore has been so altered by the construction of seawalls
or by the dumping of riprap that sand beaches occur only
in short stretches today.

The ocean shore of islands on the windward side of
the atoll facing the reef flat often have alternating
beach-beach-rock shores. Sand beaches here, however, do
not extend below the intertidal, merging with the reef flat
or rock which extends offshore.

Vegetation occurs above the high tide line on all
shores. There are no mangroves or mangrove-like terres-
trial plants extending into salt water at Enewetak. With the
exception of Biken and the five southwestern islands, the
vegetation has been extremely altered.

Enewetak, Medren, and Japtan Islands are residence
islands. Houses were constructed and other buildings were
converted during the Enewetak cleanup. These three
residence islands, plus Ananij and the islands from Billae
through Aej, were planted with coconut palms between
1978 and 1979. Coconut palms have not been planted on
Enjebi, the second largest island of the atoll, except for an
experimental garden plot that was established in 1975 by
Lawrence Livermore National Laboratory and that contains
coconuts, pandanus, and breadfruit.

The soil of Enewetak Atoll islands consists of little
more than calcium carbonate sand and rubble (Chapter 11
of this volume). This material has virtually all its origin
from the sea and is derived from corals, calcareous algae,
foraminifera, and a wide variety of organisms producing
smaller amounts of carbonate materials. Occasionally,
pieces of pumice which have drifted to Enewetak are
found near beaches. More rarely, noncarbonate rocks, car-
ried by rafting debris such as fallen trees, are found.

Enewetak soils have very little organic matter or
nutrients. This is particularly true for the highly disturbed
islands where human activity has eliminated the normal
ground cover of vegetation and nesting birds. On normally
vegetated islands, a limited amount of organic materia! is

COLIN

tied up in leaf-litter on the soil surface, but relatively little
is actually found in the soil.

The larger islands of the atoll have good freshwater
lenses beneath them. All the islands are quite low, so the
water table lies very close to the ground's surface. It is not
necessary to drill more than about 3 m deep to hit water.
The groundwaters of Encwetak have been studied in some
detail (Chapter 4 of this volume).

MAN-MADE FEATURES
Quarries

Areas of reef flat adjacent to several islands at
Encwetak were quarried or excavated for building or road
construction purposes. A single quarried area is at the
rKjrth end of Enewetak Island adjacent to MPRL (Fig. 4).
This area was quarried during the Japanese occupation.
Because a wide area of reef flat was left seaward to
reduce wave swell entering the quarry, the Enewetak
quarry is calm during low tides and is an ideal location for
snorkling and diving. Numerous investigators at Enewetak
have taken advantage of this. The Enewetak quarry covers
about 2.75 hectares and averages about 1.5 m in depth,
with the deepest sfX)t being 3 m. The biological communi-
ties present in it are discussed in Chapters 7 and 8 of this
volume.

The reef flat at the south end of Medren Island was
also quarried. Although slightly larger than the Enewetak
quarry, little protective reef flat was left seaward of it;

therefore, it is more open to wave action from the open
ocean. A small quarry occurs at the north end of Medren
on the reef flat.

Seven relatively small areas were quarried on the reef
flat near the middle portion of Runit Island. All are well
inside the seaward margin of the reef flat and are well pro-
tected from waves at low tides.

At Enjebi, there are a few areas toward the north end
where the reef flat was quarried. There is one elongate
rectangular quarry and two small round ones. Also, on the
western side of the island are three irregular areas next to
shore, deeper than the adjacent bottom, which were prob-
ably quarried for construction of the Japanese airstrip
there during World War II.

Craters

Six craters remain from nuclear weafwns testing at
Enewetak. Three craters are the result of atomic bomb
tests. The other three are from thermonuclear weapons
tests and are roughly three orders of magnitude larger in
area and volume. Two atomic bomb craters are at the
north end of Runit Island (Fig. 5). The histories, morphol-
ogy, and subsurface geology of the Enewetak craters are
extensively discussed by Ristvct (1978), resulting from
work done by the Air Force Weapons Laboratory, Albu-
querque. Both Runit craters are about 120 m in diameter.
The most lagoonward. Cactus crater, was used for con-
struction of the Cactus crater crypt during the Enewetak
cleanup from 1977 through 1979 in which the crater was
filled with cement, contaminated debris, and soil. A

Fig. 4 Aarlai view ot the north end ot tnewetaK isiana snowing the buildings of the Mid-
Pacific Rasearch Laboratory and to the right of them the quarry on the reef fiat. [Photo by
E. S. RecM.]

PHYSIOGRAPHY

Fig. 5 The north end of Runit Island with La Crosse crater (lower right) wnd the Cactus
Crater Crypt (upper left). La Crosse crater is about 120 m across and about 10 m deep. The
Cactus Crater Crypt was built in the crater to contain contaminated soil and debris. [Photo
by P. L. Colin.]

Fig. 6 The island of Boken (north) with the Seminole crater, a small atomic bomb crater.
The island and adjacent islands In the foreground have been drastically altered by the forma-
tion of the crater. [Photo by P. L. Colin.]

COLIN

25-foot-high cap of poured cement plates covers the crypt.
The seaward crater, Lacrosse crater, was not altered dur-
ing construction of the Cactus crypt. Nolan et al. (1975)
described the distribution of substrate types in these
craters and fish assemblages occurring in them (Chapter 7
of this volume).

The third atomic crater is on the west side of Boken
Island in the north of Encwetak (Fig. 6). It is similar in size
to the Runit craters, roughly 200 m in diameter, 10 m
deep, and is connected to the sea via the reef flat.

Two of the thermonuclear craters are located between
Boken and Bocinwotme Islands (Fig. 7). The MIKE crater
is the western one, roughly 1.8 km in diameter and 56 m

The last crater is some 7 km southwest of Bokoluo,
the westernmost of the northern islands. The device was
exploded from a boat anchored over the shallow lagoon
margin. This produced a crater which excavated
northwestward in the shallow reef and reef flat but is very
open to the lagoon to the southwest. It is roughly 1.7 km
in diameter.

Other Physiographic Effects from
Nuclear Tests

A large area of reef flat and seaward reef face cleaved
away in the area north of the MIKE crater sometime

Fig. 7 Mike (right) and KOA thermonuclear craters on the northern reef at Enewetak Atoll,
photographed from 10,000 feet. The larger Mike crater Is about 1.6 km across. The island of
Boken with the small Seminole crater is seen on the left side of the photograph. The section
of outer reef face which cleaved off after the KOA test can he seen seaward of the Mike
crater at the bottom of the photograph. [Photo by P. L. Colin.]

deep. The KOA crater to the east is slightly smaller, about
1.5 km in diameter. Both blasts were detonated on islands
which disappeared with formation of the craters. A third
island, Bogairikk (not shown in Fig. 1), was largely elim-
inated with formation of the craters and is now
represented solely by remnants on the sandbar west of
Boken. The MIKE crater breaches the shallow reef into the
lagoon at the 10 to 15 m depth contour. The KOA crater
is still separated from the actual lagoon by a shallow txjt-
tom of less than 6 to 10 m depth but is confluent with the
MIKE crater on its west side. A minimum of about 400 m
of reef flat separated the MIKE crater from the op>en
ocean; a slightly greater margin exists between the ocean
and KOA crater.

between 1952 and 1958 (Fig. 7). The section of reef did
not break away as a result of the MIKE test but was split
off sometime later. About 300 m of the reef face, running
as much as 60 m inward on the reef flat, fell away, and
there is no bottom visible in aerial photos over what was
once reef flat. This represents an exposure of underlying
reef structure which is of unprecedented magnitude (see
Chapter 4 of this volume for details). Direct examination of
this scarp reveals that it is vertical to slightly overhanging
with relatively sparse benthic organisms on its upper sur-
face.

Other nuclear-produced phenomena still visible at
Enewetak include ejecta trails on the reef flat produced by
thermonuclear tests, particularly in the area of the craters,

PHYSIOGRAPHY

plus small depressions on the reef flat probably produced
by single ejecta blocks.

REFERENCES

Atkinson, M J,, S V Smith, and E. D, Stroup, 1981, Circulation
in Enewetak Atoll Lagoon, Limnol. Oceanogr . 26:
1074-1083

Emery, K. O., J. T Tracy, Jr , and H. S. Ladd, 1954, Geology

of Bikini and Nearby Atolls, U. S. Geo/ Suru. Prof Pap..
260 A. pp 1 265.

Nolan, R. S , R. R. McConnaughy, and C. R. Stearns, 1975,
Fishes Inhabiting Two Small Nuclear Test Craters at
Enewetak Atoll, Marshall Islands, Micronesica, 11: 205-217.

Ristvet, B. L , 1978, Geologic and Geophysical Inuestigations of
the Enewetak Nuclear Craters, Air Force Weapons Lab., Air
Force Systems Command, Kirtland Air Force Base, New Mex-
ico, AFWL-TR-77-242.

Chapter 4

Geologi; and Geohydrologi; of Enewetak Atoll

BYRON L. RISTVET

S-CUBED. A Division of Maxwell Laboratories, Inc.
Albuquerque, New Mexico 87198

INTRODUCTION

Enewetak Atoll is located at 162° east longitude and
11° north latitude in the Pacific Ocean. It is the north-
westernmost member of the western Ralik (Sunset) Chain
of the Marshall Islands. Enewetak Atoll is one of the larger
atolls; it is roughly elliptical in shape, with a north-south
length of 40 km and an east-west width of 32 km (Fig. 1).
The reef is cut by three passes. The Deep Channel on the
southeast side is only 1.5 km wide, but it has a depth of
55 m between Japtan and Medren Islands. The South
Channel is approximately 9.5 km wide but is only 10 to
20 m deep. The Southwest Passage is even shallower,
only 2 to 4 m in depth. Maximum tidal currents of nearly
1 m s~' in the Deep Channel and 0.5 m s~' in the South
Channel have been observed (Emery ct al., 1954). The
reef may be divided into four parts with distinct morpholo-
gies related to their positions relative to the prevailing
northeast trade winds. The parts are the windward reef on
the northeast, the leeward reef on the southwest, and the
two transitional reefs on the northwest and southeast
(Fig. 1). The reef encloses a lagoon of 920 km^ with a
maximum depth of 65 m. The lagoon has a relatively
smooth carbonate sediment bottom studded with hundreds
of coral pinnacle and patch reefs (Emery ct al., 1954).
Forty-two low-relief islands and islets composed of car-
bonate sands and gravels exist on the atoll with a total dry
land area of 6.7 km^ with the largest islands being about 1
km^ in area.

Enewetak Atoll receives an average annual rainfall of
1470 mm, mostly during August to December. Rainfall is
highly variable with annual totals ranging from 605 to
2422 mm (Buddemeier, 1981). Tides are of the mixed
semidiurnal type with a maximum range of about 1.8 m.

The purpose of this chapter is to summarize the vast
wealth of data on the geological aspects of Enewetak gath-
ered over the last 40 years.

A SUMMARY OF GEOLOGIC
INVESTIGATIONS

The history of investigations of atoll geology in general
and Enewetak Atoll in fiarticular may be divided into three
periods: pre-1946, 1946 to 1964, and post-1964. Th«
first period was one of discovery and initial exploration.
These early observations became the framework for many
hypotheses on the origin and evolution of atolls. Most of
the early studies focused on the surficial geologic features
and lacked the direct sampling of subsurface data to evalu-
ate the many hypotheses of the day. Beginning In 1946,
there was a significant increase in knowledge of atolls
resulting from a series of comprehensive scientific studies
of the northern atolls of the Marshall Islands, particularly
Bikini and Enewetak. These geologic investigations were
conducted by U. S. Geological Survey (USGS) scientists
for the U. S. Atomic Energy Commission (AEC) to estab-
lish baselines to assess effects from nuclear weapons test-
ing conducted at Enewetak and Bikini between 1946 and
1958. A vast amount of surface and subsurface geologic
data was gathered and analyzed, and the results were pub-
lished through 1964 (cf. Emery et al., 1954; Schlanger,
1963). From 1964 to the present, scientific studies have
been of two types: those which have continued to addrcs*
the problems conceptualized by earlier studies and those
which have addressed the effects of the nuclear weapons
testing at the two atolls. Enewetak Atoll continues to this
day as one of the sites of significant studies of atoll geol-
ogy, carbonate sedimentology, and organism/sedinrKnt
interrelationships.

Prc-1946 Period

The first geologic studies of the Marshall Islands were
conducted in 1816 and 1817 by Albert Chamisso. Cha-
misso, a naturalist with the Russian Von Kotzebue expedi-
tion to the northern and western Pacific Ocean, described
the reefs, islands, and lagoons of the eastern chain of the
Marshalls. For the rest of the 19**^ century the MesrshAls
were visited only by general surveying expcdftkxN (Em«T?
et al., 1954).

RISTVET

2 0

^ SOUTHWEST

\ PaSSAGE

SOUTH
CHANNEL

20 STATUTE MILES

CONTOUR INTERVAL 100 FATHOMS
DATUM IS MEAN LOW TIDE

Fig. 1 Location map of Enewetak Atoll, Marshall Islands.

Meanwhile, expeditions in other areas of the world
were contributing to an understanding of atoll geology.
Darwin (1842), during the voyage of the HMS Beagle from
1831 to 1836, studied reef building organisms and reef
morphologies. He established a three-fold classification of
reefs that is still used today: fringing reefs, barrier reefs,
and atolls. Darwin (1842) integrated his findings into a
theory of atoll formation on subsiding island foundations

with antecedent fringing and barrier reef stages. However,
other workers (cf. Agassiz, 1903; Daly, 1915; and Gar-
diner, 1931) later proposed alternate theories postulating
that atoll reefs grew upwards from still standing submarine
summits of various origins.

By the turn of the century, direct subsurface sampling
became a paramount issue to understanding the origin of
atolls. Funafuti Atoll, in the Ellice Islands 2400 km south

GEOLOGY AND GEOHYDROLOGY

of Enewetak Atoll, was the site of the first sampling well
drilled on an atoll (David ct a!., 1904). This well, drilled
from a ship in the lagoon, penetrated 337 m of carbonate
sediments, demonstrating the great thickness of atoll reef
sediments.

A major review of the theories of atoll formation was
written by Davis (1928). Davis carefully evaluated the data
and hypotheses; his evaluation supported Darwin's (1842)
subsidence theory as correct and rebutted alternate
theories. Although Davis (1928) rejected Daly's (1915) the-
ory that glacial period sea level histories resulted in atoll
formation, he did enter them as an important new element
to consider in evaluating the geologic history of atolls.

Before 1946, atolls of the Marshall Islands provided lit-
tle evidence for the aforementioned theories. During the
period of 1918 to 1944, the Marshalls were under the
control of Japan; although Japanese scientists conducted
studies on the atolls, much of the resulting data are not
readily available (Emery et al., 1954). Stearns (1945) made
some general comments on possible battle damage to the
reef of Enewetak Atoll following the American occupation
in 1944.

In summary, at the end of 1945, only a small body of
data existed on atoll geology. The general locations and
morphologies were described, and the types of reef-
building organisms and their environmental requirements
were known in a general sense. Conclusive evidence on
atoll formation had not been found, and sparse data
existed on subjects such as atoll foundations, ages, lagoon
and outer slojDe sediments, reef zonations and productivi-
ties, and ecology.

1946 to 1964 Period

A period of intense scientific study on the northern
Marshall Islands began in 1946 to establish baselines from
which damage could be assessed from the U. S. Nuclear
Weapons Testing Program. Bikini Atoll was chosen to be
the site of the first nuclear weapons effects tests conducted
by the United States. Operation Crossroads, consisting of
the detonation of two atomic bombs over and under naval
ships in Bikini Lagoon, was conducted in 1946. Two
expeditions to Bikini were made in 1946 and 1947 to
study the atoll environment. The 1946 effort included gen-
eral surficial geologic studies of the rcjf, lagoon floor and
outer slopes, and a seismic refraction study of the subsur-
face structure of the atoll (Emery et al., 1954). The 1947
studies yielded much geologic information on the subsur-
face through the drilling of three holes on Bikini Island and
reef, with one hole penetrating 775 m of carbonate sedi-
ments (Ladd et al., 1948). In 1950 additional seismic
refraction studies were completed at Bikini and the adja-
cent Sylvania Guyot and the southern part of Kwajalein
Lagoon (Dobrin and Perkins, 1954; Raitt, 1954). An
aeromagnetic survey of Bikini Atoll (Keller, 1954) was also
completed.

Nuclear testing began at Enewetak Atoll in 1948 with
three events of Operation Sandstone. Shortly thereafter.

the USGS began a series of geological and scientific inves-
tigations again to establish baselines to measure the effects
of the nuclear detonations. In 1950 four shallow holes
were drilled by the AEC in the reef on the seaward side of
Engebi (Enjebi) Island to locate a suitable rock quarry
(Ladd and Schlanger, 1960). In 1951 the AEC drilled 17
shallow holes on six different islands for soils engineering
studies related to the construction of structures for the
nuclear testing. The AEC also drilled three deep holes on
the atoll in 1951 and 1952 under the technical guidance
of the USGS: K-IB was drilled to 390 m on Engebi
(Enjebi), F-1 was drilled to 1411 m on Elugelab, and E-1
was drilled to 1287 m on Medren Island. Both F-1 and
E-1 reached volcanic basement with 5 m of olivine basalt
being recovered from E-1. The confirmation of a basaltic
foundation beneath Enewetak Atoll substantiated Darwin's
subsidence theory of atoll formation (Ladd et al., 1953).
Drill holes K-IB and F-1 were subsequently destroyed
during nuclear tests, but the E-1 hole is still open to at
least 609 m (Daniels et al., 1984).

After 1952 field study of the geology of the northern
Marshall Islands was reduced significantly, although the
nuclear testing continued through August 1958. The con-
tinued availability of Enewetak for future field studies was
ensured by AEC's establishment in 1954 of the Enewetak
Marine Biology Laboratory, now known as the Mid-Pacific
Research Laboratory. The AEC completed some additional
shallow drilling in 1953 and 1956 for soils engineering
(Pratt and Cooper, 1968), but no more drilling for geologic
study was completed until 1971. However, the vast
amount of field data and samples yielded in the 1948 to
1952 efforts were studied and evaluated through 1964.

Formal presentation of the completed studies was com-
piled in the USGS Professional Papers 260 Series com-
pleted in 1964. This 28-paper series comprises the most
comprehensive single body of geologic, geophysical, and
oceanographic data ever assembled on a group of atolls.
Much of the rest of this chapter will draw heavily on the
data presented in these papers.

A major pap)er by Emery et al. (1954) is a comprehen-
sive study of the surface geology of Bikini, Enewetak, and
nearby atolls. It also presents data on the sediments of the
lagoons, reefs and islands, reef morphologies and
lithologies, and coral zonations of different reefs as well as
many other topics. Also presented in the paper are the
lithologic sections for the deep holes drilled on Bikini. Sub-
surface zones of calcitic limestones are described which are
overlain and underlain by aragonitic sediments. These lime-
stones are postulated to represent times of subaerial exp)0-
sure of the atoll (Ladd et al., 1948; Emery et al., 1954).

Another paper in the 260 series by Munk and Sargent
(1954) describes the variation in the spur and groove
structure of the Bikini reefs and relates them to distribu-
tion and direction of wave energy. This relationship
demonstrates that these are not relict Pleistocene erosional
forms. Wells (1954) defined ecological zones of windward
reefs in the northern Marshalls on the basis of dominant
coral faunas and compared these zonations with reefs else-

RISTVET

where. The great organic productivity of atoll reefs versus
the surrounding oceans is demonstrated by Sargent and
Austin (1954).

The subsurface geology and geophysics of Enewetak
Atoll somewhat dominates the 260 series. The penetration
of a basaltic basement by drilling and the aeromagnetic
and seismic refraction surveys indicated the presence of
volcanoes beneath Enewetak, Bikini, and Kwajalein Atolls.
Ladd and Schlangcr (1960) present the locations and drill-
ing data for the Enewetak drill holes. They conclude that
most of the near surface material to 60 m depth is uncon-
solidated, whereas deeper zones of recrystallized and
leached carbonate are postulated to represent fjeriods of
subaerial emergence of the atoll. Foraminifera were used
to establish a Tertiary biostratigraphy of the Enewetak sub-
surface and to document continuous shallow water condi-
tions in which the entire carbonate section had been de-
posited (Cole, 1957; Todd and Low, 1960). The oldest
carbonates were identified as Upper Eocene in age.

The general subsurface geology of Enewetak was
defined by Schlanger (1963), who presents detailed litho-
logic logs of the Enewetak deep holes and provides an
interpretation of the geologic history. Schlanger (1963)
noted the presence of numerous "solution unconformities"
within the Enewetak geologic column. The term solution
was used because Schlanger felt these unconformities
represented karstic surfaces.

The scientific programs in the northern Marshalls had a
stimulating effect on the academic interests in atolls. The
interest in the geology and biology of carbonate reefs is
still a dominant field of study. The geologic studies of this
period answered many of the basic questions about atoll
formation. Atolls rested on subsided volcanic foundations.
The compositions and dep>ositional environments of the
subsurface sediments were characterized and interpreted.
Specific zones of altered carbonates were identified and
interpreted to represent periods of atoll emergence and
given paleohydrologic meaning. Reef zonation and mor-
phology, as products of interacting biological aggradation
and mechanical and biological erosion, became better
understood.

1964 to Present

Geological studies conducted during this time in the
Marshall Islands have been centered on Enewetak Atoll.
Numerous studies primarily concerned with sediment/
organism interrelationships, the distribution of radionu-
clides within the atoll sediments, and geohydrology have
been conducted under the auspices of the Mid-Pacific
Research Laboratory, which is sponsored by the Depart-
ment of Energy (DOE). The Defense Nuclear Agency has
sponsored four major field programs to understand the
craters resulting from the near-surface detonation of
nuclear weapons: the Pacific Cratering Experiment (PACE),
1971 to 1972; the Exploratory Program on Enewetak
(EXPOE), 1973 to 1974; the Enewetak Atoll Seismic
Investigation (EASI), 1980; and the Pacific-Enewetak Atoll
Craters Exploration (PEACE), 1984 to present.

Until the early 1970s, studies of Enewetak geology
consisted of reviews or extensions of previous work. Gross
and Tracey (1966) used stable carbon and oxygen isotope
data to substantiate the hypothesis that the calcific lime-
stones in the subsurface were formed in fresh water
environments (Ladd et al., 1948; Ladd and Schlangcr,
1960; Schlanger, 1963). Thurber et al. (1965) performed
U/Th radiometric dating of corals of the Enewetak subsur-
face from the Quaternary [>eriod and revealed an absence
of corals dating between 6000 and 100,000 years before
present (ybp), indicating a significant hiatus in def)osition.

At Enewetak, PACE was conducted to evaluate the
influence of the shallow subsurface geology on the dimen-
sions of nuclear explosion craters. It consisted of two
phases: (1) geological and geophysical investigations of the
shallow (<70 m) subsurface of the atoll and (2) a series of
high explosive cratering experiments. A federal court order
cancelled PACE before most of the high explosive crater-
ing experiments were conducted. However, much of the
first phase was completed, and nearly 250 shallow
boreholes were completed on seven islands, with 235
being drilled on Aomon or Runit Island (Henny et al.,
1974). Most of the holes were soils engineering borings
which returned little or no sample. Sample recovery in the
cored boreholes was variable but was generally quite fxxjr.
A generalized four-layer engineering geology model for the
shallow subsurface at Aomon Island was developed by
Henny et al. (1974) using these limited samples and seis-
mic refraction survey data.

The follow-on geologic and geophysical program to
PACE was EXPOE. The objective of the EXPOE program
was to develop a model of the near surface geology of the
atoll for the nuclear crater regions in the northern islands.
Forty-six cored boreholes and 13 water sampling wells
were completed on 11 islands on the windward, leeward,
and transitional sides of the atoll, and 250,000 lineal feet
of shallow seismic refraction surveys were completed from
1973 to 1974. The EXPOE program was notable for the
excellent sample recovery: recovery of 4-in. cores of both
consolidated and unconsolidated materials averaged over
80% (Couch et al., 1975). This recovery was far greater
than any previous drilling, especially in the pxjorly and
unconsolidated near surface sediments, and allowed for a
more detailed picture of the stratigraphy and petrology of
the upper 100 m of the Enewetak subsurface than gained
in previous studies (Ristvet et al., 1974, 1977).

As will be discussed in greater detail later in this
chapter, the EXPOE findings indicate that the atoll pe-
riphery to at least 80 m depth consists of subordinate reef
and dominant back reef and marginal lagoon deposits of
the Holocene and Pleistocene ages. Five subaerial surfaces
were recognized in the Pleistocene section associated with
sea level drops during glacial periods (Ristvet et al., 1974,
1977).

The EASI field program consisted of overwater high
resolution multichannel seismic reflection surveys of the
KOA and OAK nuclear craters and the undistu''bed lagoon
off of Engebi (Enjebi) Island (Ristvet et al., 19ti0; Tremba

GEOLOGY AND GEOHYDROLOGY

et al., 1982; Trcmba, 1985) and participation in the
MPRL sponsored R/V Makali'i submersible dives in 1981.
The EASI seismic reflection profiles showed that shallow
unconformities recognized in the EXPOE drilling continued
across the lagoon paralleling the present day bathymetry.
Additional deeper reflectors at 150 and 245 m and a
series of reflectors between 320 and 365 m were noted
and compared to the unconformities described by
Schlanger (1963). It was hypothesized that the Middle
Miocene reflectors between 320 and 365 m may be a
representation of a series of closely spaced unconformities
much like the Pleistocene section described for Enewetak
and Bikini (Ristvet et al., 1974, 1977; Tracey and Ladd,
1974). Unfortunately, only deep drilling with high
core/sample recovery would resolve this issue.

The PEACE program was a two-phase program with
the objective of understanding the surface and subsurface
morphologies of OAK and KOA nuclear craters. The first
phase of the PEACE field program was performed during
the summer of 1984 and included high resolution mul-
tichannel seismic reflection, bathymetric, side-scan sonar,
and submersible studies primarily of the two cratered areas
but included some studies of atoll-wide nature (Folger,
1986). The second phase of PEACE was conducted during
the summer of 1985 and consisted of overwater drilling
into and adjacent to the two nuclear craters. High core
recovery was obtained in drill holes as deep as 490 m
beneath the lagoon floor. The PEACE drilling data are in
the analysis phase.

During this post- 1964 period, Enewetak was the site of
several geologic investigations sponsored by MPRL. Exam-
ples of these investigations include rates of calcification of
the windward reef (Smith and Harrison, 1977), studies of
Holocene sea level histories which suggest a higher than
present sea level 4000 to 2200 ybp (Tracey and Ladd,
1974; Buddemeier et al., 1975), and investigations of the
Quaternary history of the reef flat (Szabo et al., 1985).
Submersible studies of the outer slope have been con-
ducted by Colin et al. (1986) and Halley and Slater (1985)
to define the morphology of the outer reef slope.

SURFACE GEOLOGY

General

Enewetak surficial geology is best divided on the basis
of depositional environments: the outer slope, the reef, the
islands, and the lagoon.

Outer Slope

The topography around Enewetak Atoll was first deter-
mined by 85 radial and five partially complete concentric
lines of soundings made by the USS Bowditch in 1944
(Emery et al., 1954). The profiles show a steep slope of
18° to 49° from the reef edge to 450 m depth changing
to a more gentle slope of 10° between 450 and 2000 m.
Sediments collected from a profile seaward of the South

Channel showed a predominance of fine grain and
Halimeda debris to 1500 m depth (Emery et al., 1954).

In 1981, 22 submersible dives were made on the outer
slope of the southern half of Enewetak to depths as great
as 360 m (Colin et al., 1986). The outer slope was found
to be quite steep, averaging about 60° between 90 and
360 m on the windward and transitional side and slightly
greater on the leeward side. No terraces or grooves were
noted below 30 m. Vertical grooves were noted on the lee-
ward side below 150 m depth. Talus accumulations were
noted below 150 m, with significant sediment slopes being
found seaward of the South Channel below 200 m depth.
Below 90 to 100 m depth, it appeared that no significant
reef framework was being constructed. Significant quanti-
ties of sediment are being transported down the face of
the outer slope on the windward side with little or none
being transjaorted on the leeward side.

In 1984 and 1985, Halley and Slater (1985) investi-
gated the outer slope of the reef north of the MIKE
nuclear crater utilizing the research submersible R/V Delta.
Halley and Slater (1985) noted that the slope is character-
ized by three zones: (1) the reef plate, algal ridge and near
fore reef, from sea level to 16 m depth with less than a
10° slope; (2) the by-pass slop>e, from 16 to 275 m, with
slopes of 55° decreasing to 35° near the base; and (3) a
debris slop)e less than 35° below 272 m depth.

Halley and Slater (1985) also examined an exp)osed
cross section through the reef and fore reef deposits within
a rockfall scarp created by the KOA nuclear detonation.
The slump scarp exposes three stratigraphic units that are
differentiated by the surficial apf)earance: (1) a near-vertical
wall from the reef crest to 76 m that appears rubbly and is
composed mainly of coral heads; (2) a vertical to overhang-
ing wall from 76 to 220 m that is massive and fractured,
producing smooth, blocky surfaces; and (3) inclined bed-
ding below 220 m along which the slump block has frac-
tured, exposing a dip slope of hard, dense white carbonate
rock that extends to below 400 m. Caves occur in all
three units. Fore reef boulder beds dipping seaward at 30°
are truncated by the current outer slope surface, thus
revealing the erosional nature of the bypass slop)e.

Atoll Reefs

The Enewetak reefs, like those described elsewhere in
the Marshalls and other localities in the world, show a
strong zonation in bands parallel to the front (Emery et al.,
1954). These bands are defined by both coral and
coral-algal communities (Odum and Odum, 1955) and by
sediment deposition patterns (Emery et al., 1954). Differ-
ences in the zonation types are recognizable for the three
reef types: windward, leeward, and transitional. Most previ-
ous studies have concentrated on the zonation of the wind-
ward reef (Emery et al., 1954; Odum and Odum, 1955;
Wells, 1954); however, description of the leeward and
transitional reefs are presented by Emery et al. (1954).

Figure 2 presents the zonation of the windward reef.
The zones consist of fore reef, algal ridge, coral-algal, reef

RISTVET

SU3J.3N Nl Hidaa

GE0LCX3Y AND GEOHYDROLOGY

flat, and back reef flat. Figure 2 also displays the relation-
ship of the reef to the islands and lagoon. Each of the five
zones has unique biologic and geologic characteristics.
Each of these zonations provides a model for what is seen
in the subsurface. However, as will be seen in subsequent
sections of this chapter, the Pleistocene subsurface appears
to consist dominantly of subtidal deposits, whereas the
modern reef flat consists of predominantly intertidal
environments.

Tracey and Ladd (1974) and Buddemeier et al. (1975)
present evidence that the broad intertidal, rocky platform
of the modem windward reef flat consists of lithified sub-
tidal sediments implying a previous higher-than-present
Holocene sea level. The modern windward reef is an ero-
sional platform develo[)ed after a growth of the Holocene
reef to a higher sea level. Hence, there is the possibility
that the modern Enewetak windward reef flat is not a good
model to use to interpret former aggradational reef
environments seen in the subsurface.

The windward fore reef consists of an area 30 to 50 m
wide, sloping gently seaward at 10° to 15° and covered
with coral and Halimeda sp. These gentle slopes do not
exist on the leeward reef where the fore reef has 40° to
60° slopes. The same biological communities exist on the
leeward fore reef as on the windward side (Colin et di.,
1986). The fore reef extends to a depth of 30 m where
the slope rapidly steepens, and the presence of stony
corals and Halimeda declines drastically. At fairly regular
intervals along the slop>e, there are nearly straight grooves
perpendicular to the reef face. These grooves are from
2 to 3 m wide and 8 to 15 m long and are separated by
spurs 5 to 10 m or more wide. The spurs are composed
of living encrusting coralline algae (Emery et al., 1954).
The origin of the grooves and spurs has been suggested by
Munk and Sargent (1954) to dissipate the wave energy
against the reef front. These grooves often extend into the
algal ridge, especially on the transitional and leeward
edges. The fore reef appears to be a site of active reef
building with the sediments being cemented by biologic
binding and penecontemporaneous marine cementation.

The algal ridge is primarily composed of encrusting red
algae, primarily Porolithon. The algal ridge may actually
grow above the reef flat elevation to as much as 0.3 to
0.6 m above the lowest low water due to wave action
keeping the living algae wet during low water. The algal
ridge with its biological and marine cementation provides
the framework for the preservation of the back reef and
lagoonal sediments from the erosion of ocean waves
(Emery et al. 1954). Algal ridges occur on both the
lagoon and ocean sides on the leeward reef. Both of these
leeward algal ridges are poorly developed and do not rise
much above the lowest low water.

On the inner side of the algal ridge, there is a belt of
rich coral growth from 50 to 150 m wide. Stony corals
cover more than 50% of the reef surface. Shallow pools
contain most of the coral. The remainder of the zone is a
pavement of encrusting red algae. The growth forms of the
coral colonies are low or encrusting to withstand the wave

action and low tides. Corals are predominantly Acropora.
Pocilhpora, and Montipora.

Again the coral-algal zone through biological and
marine cementation provides well-cemented sediments for
incorporation into the subsurface.

The windward reef flat at Enewetak is a fairly level
rock surface that may be divided into two rather distinct
parts: (1) a barren rock surface that appears to be the ero-
sional surface of an older reef and (2) a rock substrate with
a thin veneer of organisms, primarily the articulate red
alga, Jania, giving the surface an appearance of being
covered by a mat which Smith and Kinsey (1976) dubbed
the "algal-turf."

Tracey and Udd (1974) and Buddemeier et al. (1975)
present evidence to suppxsrt a higher-than-present sea level
between 4000 to 2200 ybp. This higher sea level may
have been 1 m or more greater than the present. The ero-
sional nature of the present reef flat is postulated to be
due to the lowering of sea level to near its present datum
around 2000 ybp. Tracey and Ladd (1974) support their
hypothesis with age dates of planed coral heads in the
present windward reef flat seaward of Runit and Aomon
Islands. Additional evidence is provided by Buddemeier et
al. (1975), who through age dating show that much of the
windward reef flat seaward of Aomon Island is composed
of cemented subtidal deposits now present in an intertidal
zone, the result of a recently lowered sea level. Additional
evidence for a higher-than-present Holocene sea level
around 4000 ybp for other Pacific islands may be found in
Curray et al. (1970) and Chappell and Veeh (1978).

Despite its apparent erosional character, the present
windward Enewetak reef flat is a highly productive reef
environment in terms of the mass of carbonate sediments
produced (Smith and Harrison, 1977). The algal-ridge,
coral-algal zone, and the reef flat compose what is termed
the "reef plate" (Henny et al., 1974). The reef plate con-
sists of well-cemented rock resulting from penecontem-
poraneous biologic and marine cementation.

During PACE and EXPOE, several holes were drilled
on the reef plate seaward of Aomon and Runit Islands.
These holes, in addition to the outcrops exposed in quar-
ries on the Enewetak, Medren, Runit, and Engebi (Enjcbi)
reef flats and the outcrops exposed in the LaCrosse
nuclear crater on the Runit reef flat, show that the Holo-
cene reef plate is a lagoonward prograding wedge of well-
cemented sediments overlying unconsolidated subtidal car-
bonate sands and gravels. The seaward edge of the wedge
begins approximately at the reef plate /coral-algal zone
boundary. Within the shallow Quaternary subsurface, sedi-
ments beneath the coral-algal zone appear to be continu-
ously well cemented with depth. Beneath the reef flat, the
thickness of the wedge tapwrs from 3 to 4 m at the center
of the reef flat to <1 m at the back reef /reef flat bound-
ary (Ristvet et al., 1977).

The back reef is characterized by small to large solitetfy
coral heads of Pontes and Heliopora in a rocky to sandy
substrate. Little or no marine cementation app>ears to be
occurring, and the sands and silts have their origin from

RISTVET

sediment production and bioerosion of the reef flat. This
environment extends from the reef flat to the islands.

Atoll Islands

The present islands of Enewetak represent wave and
eoiian deposits of excess sediment production from the
reef stabilized in part by the formation of beachrock.
Islands are present on the reef except on the northwest
transitional reef. The islands are all approximately 3 to
4 m in elevation above the lowest low water. Two basic
island shapes exist for Enewetak Atoll: (1) long linear
islands that parallel the reef h-ont, such as Runit,
Enewetak, and Bokoluo and (2) the triangle-shaped islands
with the base on the lagoon side parallel to the reef front
and the point facing the seaward reef, such as Engebi
(Enjebi), Aomon, and Louj. The origin of these two island
shapes is not understood. The islands are covered with
vegetation and have fairly well-developed soil profiles.

The origin of beachrock has been the subject of several
investigations at Enewetak and other carbonate beaches in
the world. Beachrock at Enewetak is present on 30 to
40% of all beaches. The formation of beachrock appears
to be a fairly recent phenomenon with significant formation
continuing today.

The author has collected samples of beachrock at
Enewetak encapsulating World War 11 shell casings and
cables from the nuclear testing pjeriod. The origin of
beachrock was first investigated in the Marshalls by Emery
et al. (1954), who looked at interstitial water chemistry
and concluded that evaporation and heating of interstitial
seawater resulted in carbonate precipitation. Schmalz
(1971) studied the interstitial water of beach sediments on
the lagoon side of Bijire Island in 1967. He concluded that
precipitation of the dominant acicular aragonite and minor
micritic magnesian calcite cements in the interstices of the
carbonate sand was caused by the mixing of seawater with
the brackish meteoric water in the thin Gyben-Herzberg
lens. A succession of studies on the origin of beachrock
cements followed Schmalz (1971). Commonly invoked
processes for the precipitation of beachrock cements
include evaporation of seawater, mixed fresh-saline
waters, and vague types of biological involvement (Manor,
1978). Current models show that degassing carbon dioxide
from beach groundwaters appears to be the primary
phenomenon that forms beachrock (Manor, 1978).

Atoll Lagoon

The bathymetry of the lagoon was mapped in detail by
the U. S. Navy in 1944. Nearly 180,000 soundings were
made, and the results were contoured (Emery et al., 1954,
chart 5). The lagoon bathymetry is somewhat irregular due
to the presence of numerous coral knolls (patch and pinna-
cle reefs). The lagoon consists of four major bathymetric
features: (1) lagoon terrace; (2) lagoon basin; (3) coral
knolls; and (4) the reef openings.

The lagoon bathymetry shows a terrace between 15
and 22 m depth (Emery et al., 1954). The terrace borders

all edges of the lagoon except the northwest and southern
margins, where it is absent. The width is variable with 3
km being the greatest attained. The lagoon terrace is dot-
ted with numerous patch reefs. The slopes from the
islands to the terrace are gentle, averaging <2.5°. An
even gentler slope, averaging 1.25°, separates the terrace
from deep basin (Emery et al., 1954).

The main lagoon basin is a relatively flat area with
slopes of 0.10°. The greatest depths are nearly 65 m in
the northwestern half of the lagoon. The mean depth of
the basin is approximately 55 m (Emery et al., 1954).

Within the lagoon are a large number of individueJ
coral knolls or patch and pinnacle reefs. Emery et al.
(1954) reported the presence of 2293 individual coral
knolls. About 10% of the knolls rise to within 8 m of the
surface. Most have tops between 30 and 36 m depth. The
distribution of the coral knolls within the lagoon apf)ears to
be random. Seismic reflection profiles from EASl and
PEACE through knolls suggest that they are predominately
Molocene features. Nearly half of the knolls are formed
over what is interpreted to be preexisting eroded Pleisto-
cene patch or pinnacle reefs, whereas the other half of the
lagoonal coral knolls do not appear to have an antecedent
structure beneath them (Tremba, 1985; Grow et al.,
1986).

The bottom sediments of the Enewetak Lagoon were
first characterized by Emery et al. (1954) and most
recently by T. W. Menry and B. R. Wardlaw (personal
communication). Emery et al. (1954) found that the sedi-
ments consist of the following chief components: Halimeda
sand, coral sand and gravel, foraminifera sand, mollusc
shell sand and gravel, and fine debris. Fine debris was
defined as all grains <0.25 mm in diameter. Emery et al.
(1954) show the terrace to be dominated by fine debris
and the basin by Halimeda and foraminifera sand. Menry
and Wardlaw (1985) show a similar distribution but reiport
much more mud-sized (<0.062 mm) carbonate sediment
on the terraces and in the basin than Emery et al. (1954).

McMurtry et al. (1985) have investigated the magni-
tude of bioturbation of the lagoonal sediments off Runit
Island and found that the burrowing shrimp of the family
Callianassidae nearly completely mix and redistribute the
surface sediments to a subbottom depth of at least 1.5 m.

SUBSURFACE GEOLOGY AND
GEOPHYSICS

The subsurface geology of Enewetak Atoll consists of
an approximately 1370 m thick carbonate sediment
caprock overlying the summit of a basaltic volcano that
rises 5000 m above the floor of the ocean (Ladd et al.,
1953). Most o* the drill hole data for the interpretation of
the subsurface geology of Enewetak are derived from
drilling on islands or the reef flat. The PEACE program
has added data to 490 m subbottom depth on the north-
ern lagoon terrace and northwestern shallow lagoon. The
subsurface geology, as deduced from the analysis of the
borehole samples and seismic profiles, is very similar to

GEOLOGY AND GEOHYDROLOGY

the subsurface geology of Bikini Atoll (Emery et al., 1954)
and Midway Atoll (Ladd ct al., 1970).

Basement Rocks

In 1952, two deep holes (Fig. 3) reached the volcanic
basement below the carbonate sediment caprock. Hole

F-1 on Elugelab Island encountered hard basement rock at
1405 m depth. In hole E-1 on Medren Island, unweath-
ered basalt cuttings were recovered from 1267 m, and
solid basalt cores were taken from 1282.5 m to 1287 m.
The basalt was an alkali olivine basalt containing analcime
(Schlanger, 1963). The rock is similar to the late lavas of

INSERTS APPROXIMATE

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INDEX MAP

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Fig. 3 Location map of three deep AEC holes drilled in 1951 and 1952. [From Ladd and Schlanger, I960.]

RISTVET

the Hawaiian volcanoes and other islands of the central
Pacific Ocean. Kulp (1963) found the basalt to be Eocene
in age with a whole rock K/Ar radiometric date of 59 ±
2 million ybp and a pyroxene K/Ar date of 51 ±5 mil-
lion ybp.

The shape of the volcano is characterized by the two
drill holes at the north and southeast edges of the atoll,
the seismic refraction profiles of Raitt (1957) and the
recent seismic reflection profiles of the PEACE program
(Grow et a!., 1986). Figure 4 displays the subsurface ve-
locity structure of the atoll from the surface to the upper
mantle as interpreted by Raitt. Although not depicted in
Fig. 4, the uppermost velocity layer is a thin (105 m
thick), low velocity (1920 m s~^) unit detected beneath
the reef northwest of Elugelab Island. The second and third
layers have apparent harmonic mean velocities of 2440 m
s~' and 3050 m s~\ respectively. Raitt (1957) suggests
that both velocities are characteristic of partly con-
solidated calcareous sediments. The fourth through sixth
layers comprise the volcanic basement underlying the car-
bonate caprock. Finally, the seventh layer has a seismic
velocity, 8.1 km s~\ characteristic of the upper mantle.
Grow et al. (1986) show, in seismic reflection profiles, the
top of the volcanics to be a relatively flat surface with only
minor topograpy.

Carbonate Rocks

Figure 5 displays Schlanger's (1963) generalized
interpretation of the subsurface of Enewetak Atoll based
on the three deep holes drilled in 1951 and 1952. Also
used for comparison is the interpretation of the subsurface
of Bikini (Emery et al., 1954) which shows a strong simi-
larity in the vertical extent of these zones for both atolls.
Schlanger (1963) recognized that beneath both Enewetak
and Bikini, there are zones characterized by the presence
of fossil molds and solution features (leached and altered
sediments) alternating vertically with zones containing pri-
mary skeletal aragonite (unaltered sediments) separated by
relatively sharp boundaries. Schlanger (1963) termed the
upper surfaces of the leached zones "solution unconformi-
ties," because they resemble karstic surfaces. Ladd et eil.
(1948) suggested that the leached zones at Bikini
represented periods of atoll emergence and exposure of
the marine sediments to subaerial conditions. The unal-
tered zones were believed to represent sediments that
were never emergent.

Schlanger (1963) identified three major solution uncon-
formities in the subsurface of Enewetak at depths of
850 m (Early Miocene), 335 m (Middle Miocene), and
90 m (Pleistocene) below the surface (Fig. 5). Schlanger
(1963) also described an interval of partially leached and
altered sediments within the upper 90 m of the Enewetak
subsurface. However, due to limited sample recovery he
was unable to identify a solution unconformity within this
interval. He did conclude that at least one additional
p>eriod of atoll emergence had occurred during the Pleisto-
cene following the emergence related to the major solution
unconformity at 90 m depth.

The high percentage of recognizable fossil material in
the three deep drill holes allowed Schlanger (1963) to
interpret the depositional environments of the sediments.
Figure 6 presents the interpreted paleoecology of holes
E-1 and F-1 at Enewetak and 2A-B at Bikini. The sec-
tion of Eocene fore reef deposits in hole F-1 between
1280 and 1385 m represents outer slop)e deposits contem-
fwraneous with near-reef, shallow-water deposits in hole
E-1 from 845 to 950 m depth. The section of fore reef
deposits in F-1 from 822 to 1280 m has no counterpart
in E-1. Schlanger (1963) interpreted this as evidence that
the earliest reef building at Enewetak began on the
southeast side of the atoll near Enewetak and Medren
Islands of today. Reef production and p>ossible erosion of
the southeast reef during the lower Miocene emergence
resulted in the wedge of fore reef sediments seen in F-1.
The seismic reflection studies of Grow et al. (1986) app>ear
to confirm a prograding reef front from southeast to
northwest starting in the presumed Eocene sediments and
continuing to the Middle Miocene unconformity.

Post-Eocene carbonate sediments are all of shallow-
water origin as sampled in the three deep holes, the
EXPOE holes, and the PEACE holes. By the Middle
Miocene unconformity, the location of the reef tract
apF>ears to have been very close to the present position of
the modern day islands (B. R. Wardlaw, personal com-
munication). In the substantial time represented by the
upper 335 m of carbonate sediments, the reef tract has
only migrated seaward 200 to 300 m. Deposition of
shallow-water sediments under conditions of slow atoll
subsidence continued through the Middle Miocene (Cole,
1957). However, the presence of several disconformities
noted in the PEACE drilling from 335 to 490 m
(T. W. Henry and B. R. Wardlaw, personal communica-
tion) and Schlanger's (1963) reported presence of
recrystallized limestone from 603 to 650 m suggest some
periods of atoll emergence.

Minor amounts of dolomitized limestone were noted
within the Eocene stratigraphic section in both F-1 and
E-1 and below the assumed Lower Miocene solution
unconformity in F-1 (Schlanger, 1963). The dolomite
appears as protodolomite replacing calcite. Schlanger
(1963) felt that its origin may have resulted from the
alteration of high magnesian coralline algae. Other
hypotheses have been proposed including dolomite forma-
tion in the mixing zone of meteoric groundwaters with sea-
water during atoll emergence and/or formation from
hyf)ersaline conditions in a restricted shallow-water environ-
ment penecontemp)oraneous with dep>osition. Sailer (1984)
presents new evidence using stable Sr isotope data that
the Enewetak dolomite precipitated from normal seawater
significantly following deposition at burial depths greater
than 900 m.

The 335 m unconformity described by Schlanger
(1963) indicates a major emergence occurred after the
depKJsition of Middle Miocene sediments. Ristvet et eJ.
(1980) postulated, on the basis of the EASI seismic reflec-
tion profiles, that the 335 m solution unconformity con-

GEOLOGY AND GEOHYDROLOGY

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EXTRAPOLATIONS AND INTERPOLATIONS

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RISTVET

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GEOLOGY AND GEOHYDROLOGY

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sisted of several closely spaced unconformities similar to
the Pleistocene section described by Ristvet et al. (1977).
Preliminary results from the PEACE program show the
presence of at least four subaerial surfaces between 310
and 350 m subbottom depths. This suggests that the Mid-
dle Miocene may have had episodic continental glaciation
conditions similar to those well documented for the
Pleistocene/Late Pliocene epKjchs. At least two of these
unconformities show karstic features suggesting relatively
long periods of subaerial exposure (B. R. Wardlaw, per-
sonal communication).

Resubmergence of the atoll occurred in Tertiary / time
with the deposition of shallow-water sediments. From 210
to 252 m the sediments represent very organic-rich, nor-
mal lagoonal, or shallow-water deposits. Preliminary
PEACE data suggest that the sea level did not fall during
this depositional interval. Lignitic material is scattered
throughout the interval. Leopold (1969) reported a polli-
niferous interval from the early deep holes from 205 to
270 m. This interval is interpreted as being a time when
the atoll had rather large islands and large mangrove
swamps developed (B. R. Wardlaw, personal communica-
tion). At a depth of more than 210 m, the sediments indi-
cate normal shallow-water deposition and a return to the
small island configuration.

Schlanger (1963) describes the presence of a major
solution unconformity at a 90 m depth. Preliminary
PEACE data show this to be the top of the Pliocene
(T. M. Cronin, personal communication). A second
subaerial exposure surface is recognized approximately
15 m below the 90 m solution unconformity.

The PEACE drilling program has provided nearly con-
tinuous sampling of the upper 490 m of the Enewetak sub-
surface near its northern and northwestern lagoonal edges.
Unfortunately, results of this recent drilling program are
still forthcoming. Preliminary results of the PEACE drilling
confirm the general interpretations made by Schlanger
(1963); however, they provide a significant increase in the
detailed understanding of the post-Lower Miocene strati-
graphic section unavailable to Schlanger (1963) due to the
poor sample recovery of the 1951 and 1952 drilling. It is
anticipated that the PEACE results will lead to redefinition
of the biostratigraphy, based both on foraminifera and
ostracods of the post-Eocene of Enewetak and the Pacific
in general. A detailed understanding of the Enewetak sea
level history will also be forthcoming as well as additioneil
insight into the processes of the diagenesis of carbonate
sediments.

The Quaternary subsurface of Enewetak is well-defined
by the data obtained during the EXPOE drilling and is now
further supplemented by the PEACE drilling. Five major
unconformities were recognized by Ristvet et al. (1974,
1977); Goter (1979); and Szabo et al. (1985). Figure 7
presents an ocean reef to lagoon cross section across
Engebi (Enjebi) Island constructed from the logs of EXPOE
drill holes (Couch et al., 1975) and supplemented by data
from the geologic rcdescriptions of several of these holes
for the PEACE program (B. R. Wardlaw, personal com-

munication). Each of the five unconformities represents
paleosubaerial exposure surface and is marked by the pres-
ence of paleosols (terra rosa type), soil base features (lam-
inated crusts, rhizoconcrctions, etc.), and/or prominent
changes in the mineralogical and chemical compwsition and
cementation of the sediments. These five unconformities
arc within the upper zone of leached and altered sediments
described by Schlanger (1963). Because of the excellent
core recovery during the EXPOE drilling, the identification
of these Quaternary unconformities was easily made.
Szabo et al. (1985) have dated three of the first four litho-
somes using a variety of radiochemical techniques. These
ages are shown in Fig. 7,

The first unconformity at approximately 10 m depth is
the Holoceiie/Pleistocene contact. Radiocarbon dates indi-
cate that the Holocene sea transgressed the emergent plat-
form reef by about 8000 ybp. The reef grew rapidly
upward (about 5 to 10 mm yr~') until approximately
6500 ybp. Following 6500 ybp, vertical growth slowed to
about 0.5 mm yr~' prompting lateral development of the
reef (Szabo et al., 1985; Tracey and Ladd, 1974). As pre-
viously discussed, sea level may have been nearly 1 m
higher than present between 4000 and 2200 ybp. Current
relative sea level rise at Enewetak may be near that of the
long-term subsidence rate of 0.02 to 0.04 mm yr~' (Bud-
demeier et al., 1975). Smith and Kinsey (1976) estimate
that the present Enewetak reef has potential for upward
growth of approximately 1 mm yr~^ The difference in
growth potential versus modem relative sea level rise may
explain why the reef plate is prograding lagoonward as
noted by Ristvet et al. (1977) for the windward reef off
Runit and Aomon.

Pleistocene rocks in the lithosome directly below the
first unconformity are dated at 131,000 ± 3000 ybp by
Szabo et al. (1985) and 100,000 to 120,000 ybp by
Thurber et al. (1965). There is also a significant change in
the mineralogical and chemical composition of the sedi-
ments below this first unconformity versus the Holocene
sediments above. Ristvet et al. (1974) document the near
total loss of high-magnesian calcite below this layer and
significant decreases in the whole rock trace element con-
centrations of Mg, Fe, and Mn. Calcitic vadose and
phreatic carbonate cements are first encountered in this
lithosome.

The development of the unconformities and the associ-
ated diagenesis of the underlying carbonate sediments are
the result of relative sea level changes during the past.
Periods of worldwide continental glaciations cause a lower-
ing of sea level. At Enewetak during the Quaternary, this
may have been in excess of 100 m (Walcott, 1972) during
each major glacial advance. During these periods of sea
level lowstands, the Enewetak Lagoon is above sea level
and the atoll becomes a large, high carbonate island,
resulting in severe changes to both the hydrologic regime
and sediment production of the atoll. Because the reefs
are subaerially exposed, only minor reef growth occurs as
a fringing reef on the outer slopes of the atoll-island. The
atoll-island undergoes subaerieJ erosion and soil develop)-

GEOLOGY AND GEOHYDROLOGY

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RISTVET

merit from colian sources. Such subaerial exposure to
meteoric waters results in the development of an extensive
Gyben-Herzberg lens within the island which is conducive
to the alteration and cementation of the sediments.

During atoll emergence, several processes acting alone
or in various combinations can produce significant modifi-
cations in those carbonate sediments exposed to meteoric
waters. Most of these processes arc dependent on the
initial dissolution of carbonate minerals into an aqueous
phase. Subsequent precipitation of calcium carbonate may
be caused by changes in carbon dioxide pressure, tempera-
ture, evaporation, mixing of waters of differing ionic
strength, and other mechanisms (Bathhurst, 1971). Precipi-
tation appears to be highly variable in both space and
time. It may be contempKsraneous with dissolution or may
involve transport over large distances.

The model prop>osed for the diagenesis of Enewetak
sediments is similar to that profxssed for other carbonate
sequences (Thorstenson et al., 1972). Meteoric waters
passing through a soil approach equilibrium with the
ambient CO2 pressure which is normally significantly
higher than atmospheric CO2. These high CO2 waters
promote dissolution of the metastable aragonite and
magnesian calcite mineralogy of recent carbonate sedi-
ments and approach equilibrium solubility. The saturated
waters at a later stage encounter an environment of lower
CO2, causing degassing of CO2 and the subsequent precipi-
tation of calcite. The process of CO2 control on
solution-precipitation of carbonates occurs within both the
vadose and phraetic zones. At standard pressures and tem-
peratures, the loss of high-magnesian calcite to calcite
generally precedes the solution of aragonite and the con-
current development of moldic porosity before the precipi-
tation of calcite.

As may be seen in Fig. 7, several [>eriods of atoll
emergence have been followed by submergence during the
Quaternary. For the Quaternary, it appears that following
each sea level rise, the new depositional environment
parallels that below the unconformity and buries it with
new sediments as the platform subsides. The processes
involved in subaerial diagenesis of the sediments during
each f)eriod of emergence are multiple upon the older
lithosome below each unconformity. In other words, for
any depth within the meteoric vadose and phraetic regime,
there is a potential for the solution reprecipitation process
to occur as many times as there are subaerial exposures
above that depth. This multiple diagenesis results in pro-
gressive increases in cementation and mineral stability with
increasing depth for at least the Quaternary section of the
Enewetak subsurface.

The Quaternary subsurface of Engebi (Enjebi) (Fig. 7)
consists of a complex mosaic of depositional lithofacies,
which have subsequently been affected by diagenetic
processes. In general, cementation increases with depth
and towards the reef within each stratigraphic unit. This
lateral change in cementation and, as shown by Ristvet et
al. (1974), corresponding changes in the rates of mineral
stabilization and trace element petrochemistry may be in

part due to (1) the occurrence of marine cements in those
sediments near the reef flat versus those deposited lagoon-
ward and (2) to diagenetic processes affecting the sedi-
ments as a function of the paleohydrologic regime and the
paleochemistry of the meteoric lens (Ristvet et al., 1977).

Shallow seismic refraction surveys were conducted on
windward, leeward, and transitional islands during EXPOE
and yielded consistent profiles for the Quaternary
Enewetak subsurface (Ristvet et al., 1977). As shown on
Fig. 7, four distinct velocity intervals exist. The velocity in
the unsaturated island sediments, Vq, is 330 to 600 m
s~'; \Ji is the velocity in saturated, unconsolidated Holo-
cene sediments and is typically about 1600 m s~^ The
velocity in poorly to moderately cemented Pleistocene sedi-
ments, V2, is typically 2500 m s ^ The V1/V2 interface
corresponds to the first unconformity. The higher velocities
of well-cemented sediments which occur on the reefward
side of the island and at depths below 60 m as inferred
from lithologic descriptions of drill holes are represented
by V3 (Ristvet et al., 1977).

The unconformities recognized by the drilling on the
atoll edges may also be followed into the lagoon on seismic
reflection profiles obtained during EASI and PEACE
(Ristvet et al., 1980; Tremba et al., 1982; Tremba, 1985;
Grow et al., 1986). Figure 8 is the interpretation of a
seismic reflection record which is a lagoonward extension
of the Engebi (Enjebi) reef to lagoon geologic cross section
shown in Fig. 7. The seismic profile is perpendicular to the
reef front and crosses the lagoonal terrace into the lagoon
basin. In Fig. 8, the first reflector/refractor corresponds to
the Holocene/Pleistocene unconformity at 15 m subbottom
depth. The reflector at 66 m subbottom depth seems to
correspond to a Pleistocene unconformity seen in the
Engebi (Enjebi) drill holes. From the PEACE drilling, it is
apparent that the deeper reflectors between 150 and
330 m correspond to lithologic changes and do not neces-
sarily represent unconformities. The 330 m reflector does
represent the top of a series of closely spaced reflectors
corresponding to the Middle Miocene unconformities recog-
nized in the PEACE boreholes. Of interest is that parallel-
ism of the reflectors to the present bathymetry. This
feature of the seismic records was noted atoll-wide for
reflectors above the Middle Miocene unconformities helping
to confirm the hypothesis that the present-day reef
environments have shown little lateral migration since the
Middle Miocene.

GEOHYDROLOGY

Studies of the hydrology of Enewetak Atoll were ini-
tiated in 1972 to evaluate possible environmental effects of
the proposed PACE high explosive craters on the ground-
water resources of the islands (Koopman, 1973). Addi-
tional studies sponsored by the DOE have been conducted
as part of a program to determine the physical, chemical,
and biological mechanisms controlling the distribution and
transport of radionuclides in the atoll environment (cf. Bud-
demeier and Holladay, 1977; Wheatcraft and Buddemeier,

GEOLOGY AND GEOHYDROLOGY

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1981; Buddemcier, 1981). An additional study was spon-
sored by the DNA (Buddemeier and Jansen, 1976) to
investigate the groundwater potential for use in the
Enewetak Radiological Cleanup.

Atkinson et al. (1981) investigated the water budget
and circulation of water in the Enewetak Lagoon and
found that essentially all the water input to the lagoonal
system comes from wind-driven transport across the wind-
ward reef. Since the windward reef crest is typically near
mean sea level, waves drive water from the ocean into the
lagoon at nearly all times. The windward reef blocks any
return flow. Atkinson et al. (1981) determined that nearly
all of the outflow occurs through the South Channel. The
Deep Channel had a balanced inflow and outflow. Other
input/output pathways, i.e., transport over the leeward
reef was insignificant in comparison to input over the wind-
ward reef and output through the South Channel. Atkinson
et al. (1981) calculated a mean residence time for lagoon
waters of 1 month with a maximum of 4 months for water
in the northeast section of the lagoon. Although water lev-
els were not directly observed, the circulation pattern
requires the existence of a net lagoon to ocean gradient
(Buddemeier, 1981).

Koopman (1973) first noted that, for the islands of
Enewetak, a significant discrepancy existed between the
calculated thicknesses of a fresh water Gyben-Herzberg
lens and that observed in trenches and borings in the field.
Koopman (1973) observed that the islands of Engebi
(Enjebi) and Aomon had only thin brackish water lenses
approximately one-tenth as thick as would be predicted
using conservative calculations. Buddemeier and Holladay
(1977) measured tidal lags in wells on Engebi (Enjebi)
Island and noted that there was a sharp discontinuity in
the plot of tidal lag time versus depth between 10 and
20 m subsurface depth. They hypothesized that the effect
might be due to a more p)ermeable aquifer below the first
unconformity of Ristvet et al. (1977). Wheatcraft and Bud-
demeier (1981) demonstrated, using tidal data from Engebi
(Enjebi) Island, that the classical Gyben-Herzberg lens
model does not describe the hydrologic system observed,
which is controlled by vertical transmission of tidal signals
from deeper and more permeable Pleistocene aquifer(s).

Buddemeier (1981) noted that total fresh water content
of island groundwater was essentially independent of island
area and radius and that the southern islands have approx-
imately 50% more fresh water volume than the northern
islands. In addition to this difference in gross fresh water
inventory, Buddemeier (1981) noted the northern islands
have thinner layers of p>otable water and more extensive
brackish water transition zones than do the southern
islands.

Buddemeier (1981) made additional tidal measure-
ments on Japtan, Biken (Rigili), Enewetak, Aomon, and
Engebi (Enjebi) Islands and concluded that significant differ-
ences were present between the amplitudes of reef and
lagoon tide stations on the falling tide resulting in a net
lagoon to ocean head. Buddemeier (1981) concluded that
this net head of water will tend to set up a lagoon to

ocean flow of water through the permeable Pleistocene
aquifer and that the amount and quality of fresh island
groundwater is controlled by the rate of lagoon to ocean
flow through the Pleistocene aquifer. The estimated lagoon
to ocean transit times are on the order of 3 to 6 years,
which corresponds well to the fresh water residence time
estimates of the islands based on inventory and recharge.
The rate of flow from lagoon to ocean dep>endency
explains why islands in close proximity to reef channels,
such as the southern islands, have greater volumes of fresh
water than others.

ACKNOWLEDGMENTS

I wish to express much appreciation to Edward Tremba
for critically reviewing this manuscript and many hours of
stimulating discussion. I wish to acknowledge the fine
technical support provided by J. MacCornack and L. D.
Evans in preparing this manuscript. I express my apprecia-
tion to the many personnel of the Defense Nuclear
Agency, Department of Energy, Mid-Pacific Research
Laboratory, University of Hawaii, Air Force Weapons
Laboratory, U. S. Geological Survey, and Holmes and
Narver, Inc., who have participated and supported the geo-
logic investigations of Enewetak Atoll for the last 40 years.
Funding for this effort was provided by the Defense
Nuclear Agency.

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RISTVET

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Chapter 5

Oceanographx; of Enewetak Atoll

MARLIN J. ATKINSON

Zoology Department. Uniuersiti; of Western Australia
Nedlands, Western Australia

INTRODUCTION

Enewetak and Bikini lagoons are large, deep atoll
lagoons. The circulation systems of both lagoons are dom-
inated by wind-driven currents (von Arx, 1948, for Bikini,
Atkinson et al., 1981, for Enewetak). However, the full
dynamics of the Enewetak circulation system is explained
by a combination of wind-driven currents, slope currents
from water input by waves, and tidal currents. The findings
at Enewetak indicate that the internal circulation and flush-
ing of deep atoll lagoons is affected by atoll morphology
and local wave and tidal conditions, features which in gen-
eral control circulation in shallow atoll lagoons (Mllliman,
1967; Gallagher et al., 1971; Henderson et al., 1978;
Ludington, 1979).

This chapter begins with the general oceanography of
the northern Marshall Islands and then concentrates on the
oceanography of Enewetak Lagoon. The oceanography of
Bikini Lagoon and vicinity have been well studied com-
pared to Enewetak. In this chapter frequent comparisons
are made between Enewetak and Bikini.

NORTH EQUATORIAL CURRENT

Enewetak resides in the North Equatorial Current. In
the region of the Marshall Islands, the current Is between
6° to 8° and 15° to 17° north latitude. The southern
boundary of the current moves northward with the sun
during northern hemisphere summer and shifts back
toward the south In winter. The current has a general
westward drift between 20 to 50 cm s"^ Surface water is
isothermal to 75 m and varies seasonally between 26° and
29°C. The main thermocline is between 150 and 300 m
with a temperature of 10°C at 300 m. By 1500 m the
temperature drops to 3°C.

Between the region of 3° to 11°N the salinity Is rela-
tively low (34.1 to 34.5 °/oo) reflecting the annual net

rainfall in the region and the eastern flow of the Equatorial
Countercurrent. Higher salinities occur to the north of 11°
(the latitude of Enewetak) due to increased evaporation.
Isohalines show development of Intermediate Water about
11°N. Figure 1 shows the temperature-salinity relation-
ships of the Western North Pacific Central Water and
Pacific Equatorial Water in the region of the northern
Marshall Islands (Barnes et al., 1948). The solid lines In
the diagram indicate the temperature-salinity correlations
at different latitudes; at 20°N the water is all North Pacific
Water and at 4°N it is all Pacific Equatorial Water. The
Insert In the diagram shows the depth of transition zones
between the two water masses. Enewetak Atoll resides In
the region where the transition zone is above 200 m and
is only 50 m thick (Barnes et al., 1948).

Dynamic topography near Enewetak has never been
measured. However, data collected during the Operation
Crossroads project and by the Japanese indicate dynamic
topography to be complex near Bikini, with the presence
of eddies northwest and northeast of the atoll (Barnes et
al., 1948). Rather permanent eddies probably exist near
Enewetak because they do for other islands (Hamner and
Hauri, 1981). The complexity of dynamic heights suggests
that currents near the atolls may vary in both sp)eed and
direction.

WAVES AND TIDES

Waves formed by the northeast trade winds break on
the northern and eastern reef perimeters of the atoll. This
constant pounding of the fore-reef shapes the spurs and
grooves on the windward side. For Bikini Atoll, Munk and
Sargent (1949) used wind data to calculate wave direction,
wave height, and wave energy. The spur and groove sys-
tems on the windward side of Bikini dissipate 95% of the
calculated wave energy as frictlonal heat and channel 5%
of the energy upward to maintain a head of water on the
reef flat. The head of water establishes water flow from
the ocean to lagoon across the windward reef flats. Waves
within the lagoon are generated by local wind patterns and
have little Influence In shaping the reef structures, but they
do Influence sand transport.

ATKINSON

Salinity %

20'

34.0

15'

a
E
.*

10<

35.0

— I —

crt26

4°N /' O^^'

/ ff/ // 10°N /Vo /

Ot 27.0

600 M
-800 I PE

Fig. 1 Temperature-salinity relationships of Western North Pacific Central Water (WNPC) and
Pacific Equatorial Water (PE) in the region of the northern Marshall Islands. The solid lines show
the temperature-salinity relationship at different latitudes. The insert shows depth of transition
zone between the water masses. [Drawn from Barnes et al.. 1949.]

Breaking waves on the fore-reef and the back-reef
determine sand transport in the following ways Cross-reef
currents carry sand from the fore-reef and the reef flat to
the lagoonward rim of the reef, building and eroding
islands. Ephemeral sand spits develop on the margins of
the islands; this sand is sorted and distributed by long-
shore transport from lagoon waves and back-reef currents.
Two general patterns of sand grain size have been deter-
mined for Bikini (Emery et al., 1954): (1) grain size
increases across the reef flat from ocean to lagoon, then
inside the lagoon, grain size decreases until a depth of
approximately 15 m is reached; and (2) grain size
decreases from the middle of the seaward beaches toward
the ends of the islands and decreases from each end of the
island to the middle of the lagoon beach.

Two processes are apparently responsible for the distri-
bution of sand: (1) high energy cross-reef currents carry a
large suspended load, depositing sand as they slow down;
and (2) the continual breaking of seaward and lagoonward
waves on the islands transports sand along the shore. The

high energy currents are formed from oceanic swells
breaking on the fore-reef, and the long-shore currents are
formed by lagoon wind waves breaking on the beach. In
the future, sand transport by currents at Enewetak could
be studied as a function of wind speed and direction, surf
height, and swell direction.

The tides at Enewetak Island are usually in good agree-
ment with the U. S. Navy Tide Tables. However, lagoon
and ocean tide records show differences in amplitude, tim-
ing, and tide curve shape. When the reef is awash at
Enjebi, wave setup produces ocean tides with a mean
water level 0.3 to 0.5 m above the mean lagoon level; at
Enewetak Island, the differences are small but significant
(Buddemeier, 1981) (Fig. 2). Buddemeier also analyzed
long-term differences between the Japtan gauge and a
lagoon gauge at Biken (see Fig. 2 for location). His analy-
sis showed that while the tide records were similar in
amplitude and frequency composition, the Biken highs are
broadened and the lows narrowed by about 1 hour. Based
on an average difference in tide elevation betwfeen Japtan

OCEANOGRAPHY

Enewetak
Isl.

Fig. 2 Location of islands and currents. See text for discus-
sion. [From Atkinson et al. with permission.]

reef flats are shallow (0 to 2 m deep), and the deepest
part of the Deep Entrance is about 57 m. Because the sur-
face North Equatorial Water is well mixed to a depth of
75 m, water flowing into the lagoon, either over the reefs
or through the channels, is well-mixed ocean surface water.
Salinity, temperature, dissolved inorganic plant nutrients,
and dissolved carbon dioxide suggest little stratification
within the lagoon water (Table 1). For the data at hand,
surface water (0 to 10 m) appears to be slightly cooler
(0.2°C) and less saline (0.06 °/oo) than deeper water.
During the data collection period (July 26 to August 16,
1974), the weather was unusually rainy and cool
(S. V. Smith, personal communication). August is a period
of low wind; therefore, stratification should occur most
dramatically during this month, yet no major stratification
is evident in these data. There is only a slight indication of
the rain in the surface water. Nutrient data collected by
S. V. Smith and M. J. Atkinson during June 1979 in the
lagoon and passages also showed no vertical structure.
Several detailed nutrient profiles taken between 0 to 2 m
above the bottom, at 10 cm intervals, revealed extremely
low and unchanging concentrations. Vertically averaged
phosphate and nitrate-nitrite concentrations are contoured
and suggest a weak minimum toward the center of the
lagoon (Fig. 3).

and Biken, Buddemeier estimated that the water level is

about 6.5 cm higher at Biken than at Japtan. CURRENTS

LAGOON WATER

The lagoon at Enewetak is well isolated from the gen-
eral westward flow of the North Equatorial Current. The

Cross-Reef Currents

Cross-reef currents involve shallow flow over the wind-
ward and leeward reef margins of the atoll. The area of

TABLE 1
Chemical Data for Ocean and Lagoon Water*

Temp

(°C)

Sal

("/oo)

Total

alk

(eq m^

PO«

NO,

NH^

(mmoles m )

Total

CO2 Pco,

(moles m ^) (uatm)

Ocean

Surface

29.5

34 30

2.29

0.12

0.21

0.50

8.31

1.88

297

S. Dev.

0.2

0.04

0.03

0.05

0.12

0.20

2.3

0.02

0.04

Lagoon

Surface (0.10 m)

29.6

34.22

2.29

0.14

0.15

0.37

3.4

829

1.89

313

S. Dev.

0.5

0.19

0.05

0.08

0.21

1.9

0.04

125

117

118

116

111

101

118

116

Mid-depth (10 to 30 m)

29.8

34.28

2.31

0.13

0.28

3.4

8.25

1.90

311

S. Dev.

0.5

0.08

0.05

0.04

0.07

0.17

1.7

0.04

0.03

Deep (30 to 50 m)

29.7

34.28

230

0.15

0.18

0.27

8.27

1.91

336

S. Dev.

0.3

0.08

0,04

0.05

0.12

0.19

1.8

0.03

'Data collected by S. V. Smith, July and August 1974.

ATKINSON

(a)

(b)

Fig. 3 Phosphate (a) and nitrate (b) contours for lagoon
water. Concentrations in mmole m^'.

windward cross-reef currents is shown in Fig. 2 by horizon-
tal lines along the eastern boundary of the atoll. These
currents are a result of breaking waves on the windward
reef; they vary in response to surf height (and therefore
regional wind patterns) and tide height. From Enjebi to

Enewetak Island (Fig. 2), water crosses the reef from the
ocean to the lagoon in a direction approximately perpen-
dicular to the reef front. The windward cross-reef currents
do not reverse direction, flowing from lagoon to ocean.
The current speed ranges from 10 to 150 cm s ^'. These
currents range in volume transport from 0.05 m s ' per
meter of reef front during low tide and low surf to about
1.5 m'^ s ' during high tide and high surf. A mean volume
transport value of 0.56 m s~ m~ was calculated; this is
equivalent to 6.6 X 10 m per tidal cycle across the
windward reef (a tidal cycle is used to facilitate comparison
with other volume transports). The volume transport of the
entire windward cross-reef current probably varies by a fac-
tor of 2 to 3. Winter tropical storms drive water over the
reef in massive amounts, building and eroding atoll islands.

The area of leeward cross-reef currents is represented
by vertical lines in Fig. 2. These currents do not flow in
any well-developed pattern. Transport along the leeward
reef, rather than across it, is common. During a period of
high surf from a north swell, S. V. Smith and
E. D. Stroup (January 1976, unpublished data) measured
inward flowing currents along the northwest leeward reef.
Current speeds and volume transports at 10 different loca-
tions ranged from about 15 to 50 cm s~ and from 0.15
to 0.57 m'^ s^'m"\ respectively. Significant inflowing and
outflowing currents were measured in the region north of
Kildrenen Island and south of the Southwest Passage
(Fig. 2). Noshkin et al. (1974), using surface concentra-
tions of 2^*Pu, 239pu, ^'*°Pu, and ^^^Cs, have also shown
that, during winter high tide periods, significant amounts of
oceanic water enter the lagoon across the northwest and
southwest leeward reef.

Dye releases on the leeward reef flat demonstrate a
slow drift either oceanward or lagoonward over the lee-
ward reef margin A maximum value for oceanward flow
might be the speed of the net oceanic drift of the lagoon
surface current. A characteristic current speed over the
entire leeward reef might be about 50% of the lagoon sur-
face current speed. Much of the reef margin bares at low
tide, so an estimate of the average depth of the reef is
near 50% of the mean tidal range, or 0.42 m. The net
transport of water out of the atoll over the leeward reef
margin is estimated to be 0.4 X 10^ m'' per tidal cycle
(i.e., only 6% of the windward reef input).

Channel Currents

The Deep Entrance current (Fig. 2) reverses approxi-
mately every 6.2 hours, with the tide. The current speed
ranges from 0 to 80 cm s~\ increasing from zero to a
maximum in about 3 hours and decreasing to slack water
in another 3 hours. The period of slack water in the chan-
nel is no more than a few minutes. The direction and
speed of the current are nearly constant throughout the
water column. The volume transport of the Deep Entrance
current varies between neap and spring tides. On May 31,
1979, near maximum spring tide, the current transported
3.0 X 10* m'^ per 6.2 hours over an entire tidal cycle.

OCEANOGRAPHY

Surface and deep drogues placed in the channel on a
flooding tide reversed on the ebbing tide and returned to
near their original position. It was estimated that the net
volume transport of the Deep Entrance is approximately
zero over a tidal cycle.

The South Channel (Fig. 2) has a nearly continuous
outflow. During flood tide, surface water drifts westward
across the channel; on ebb tide, the surface current turns
from westward, to southwest, to south. The surface water
in the South Channel tends to move westward as wind
drift, while water below a depth of 5 m moves southwest
to south, depending on the tide condition. The current
speeds range from 8 to 30 cm s^'. Based on 20 drogue
measurements and dye releases over a complete tidal
cycle, the average outflow was estimated to be 6.9 X 10
m'^ per tidal cycle; this represents 75% of the total lagoon
surface current volume transport and 105% of the
estimated water flowing inward over the windward reef
flats (Table 2, and material presented later).

The Southwest Passage is a shallow break in the lee-
ward reef, yet it has a reversing current similar to the
Deep Entrance (Fig. 2). The calculated volume exchange

between ocean and lagoon is approximately 0.8 X 10* m^
per tidal cycle. Because the currents are reversing (see pre-
vious discussion), the net outflow through these channels is
small in terms of the water budget. The calculation of
volume transport of water over the entire leeward reef
includes this net outflow through this passage.

Lagoon Currents

Currents of the central lagoon may be characterized by
a surface current, a mid depth current, and a deep current.
The currents are distinguishable by their characteristic
speed and direction The water column in Enewetak
Lagoon is nearly isohaline and isothermal; salinity ranges
0.20 °/oo at most (average near 34.4 °/oo), and tempera-
ture varies by no more than 0.5°C (annual range, 27°C to
29°C).

The surface current is wind-driven. The general surface
drift is southwesterly, or downwind (Fig. 4). The spatial
and temporal variations in the current directions are
considerable. In the central lagoon, drogues move south,
west, and north, appearing to respond to the wind direc-

TABLE 2
Water Budget; Estimates of Mean and Range

Current

Transport (range)

10* m^ per 12.4 hour

(+ is to lagoon;

— is from lagoon)

Bases for calculation

Comments on current

Windward cross-reef

Leeward cross-reef

Deep Entrance

South Channel

Southwest Passage

Surface

Mid-depth

Deep

-6.6 ( + 2 2 to +198)

-0.4 (0 to -0.8)

Net = 0 (-1.0 to +1.0)
(3.0 X 10* m^ transport
each way)

-6.9 (-4 5 to -8.5)

Net = 0 (-0.2 to -0.2)
(0.8 X 10* m^ transport
each way)

9.2 (3 to 30)

8.6 (unknown)

2.2 (unknown)

0.56 m s ' m ' reef front
27,000 m open reef front

0.05 ms"'

0.4 m (half tide range)

47,000 m open reef front

0 40 ms^'

34,000 m^ (cross-sectional area)

0.15 (0.07 to 0.23) m s"'
145,000 m^ (cross-sectional

area) (2/2)"* (conversion to

normal direction)

0.40 ms"'

9,000 m^ (cross-sectional area)

0.06 m s~'

10 m X 34,600 m (maximum cross-
sectional area at 5 m depth)

0.03 m s"'

20 m X 32,000 m (maximum cross-
sectional area at 20 m depth)

0.01 ms"'

18 m X 28,000 m (maximum cross-
sectional area at 39 m)

June 21-29, 1971
continuous inflow.

Variable flow. None
to fast channel
currents.

Reversing. Typical

tidal currents 0 to 0.80 m s"

Pulsing, continuous
outflow.

Reversing. Typical
tidal currents.

Variable. Functional
to wind speed

Variable. Functional
to wind speed.

Variable. Functional to
windward cross-reef inout

ATKINSON

NE
TRADES

//^///

Fig. 4 Lagoon surface currents from drogue data. Arrows represent smoothed drogue tra-
jectories over varying lengths of time; they arc not vectors. Some drogue runs were made
during calm or variable wind. [From Atkinson et al. with permission.]

tion of the previous 6 to 12 hours. Drogue paths over 6
hours showed no rapid changes in directions; however,
they often traced slight curves, suggesting they were
slowly changing direction with the wind.

The speed of the surface current is approximately 2%
of the wind speed (Fig. 5). Data for Fig. 5 were taken
from days when both the average wind direction and the
average wind speed had small standard deviations. Correla-
tions between wind and current speed and wind and
current direction for all data are poor, probably because
cross-reef currents and tidal channel currents influence the
surface current at least 5 km into the lagoon.

The surface current moves in a layer which varies from
5 to 15 m thick. The average thickness of the surface
layer is approximately 10 m. Downwind volume transport
of the surface layer is approximately 9.2 X 10 m per
tidal cycle (Table 2). Von Arx (1948) reported that the sur-
face current at Bikini is 5 to 20 m thick and changed
depending on the wind conditions.

The mid-depth current lies between 10 and 30 m in
depth This current generally flows northeastward, oppo-

Current = 0.0246 -^ 2.06 (Wind)
r2 = 0.96

_L-

-o

_1_

0 1 2 3 4 5 6 7

Wind speed (m s-i)
Fig. 5 Surface current speed as a function of wind speed for
the center of the lagoon.

OCEANOGRAPHY

site the surface flow, at speeds of 2 to 4 cm s The
volume transport of this current is approximately 8.6 X
10* m^ per tidal cycle (Table 2).

The deep current flows southward between 30 and 50
m. This current is slow, ranging from 0.5 to 1.5 cm s
Drogues in this current were followed for up to 10 days;
while the cumulative direction and speed were consistent
and predictable, a 6- to 12-hour east-west variability
("slosh") was noticed in their movement. This motion was
attributed to flow around the lagoon pinnacles and/or tidal
pulsing. The volume transport of the deep current is
approximately 2.2 X 10* m^ per tidal cycle through a
cross section near the middle of the lagoon (Table 2).

Vertical Current Profiles in Lagoon

Figures 6a and 6b are photographs of vertically
suspended fluorescine dye dispensers. These profiles reveal
the spiral current structure of the lagoon water. The pro-
files show that the deeper currents are offset to the right
(clockwise) from the shallower currents. Figure 7 is a
graphic summary of the vertical dye profiles in the lagoon
The number at the end of each arrow is the depth in
meters of the observation. The arrows have no magnitude
because current speeds were not determined. In all stations
across the lagoon, from Runit to West Spit (Fig. 7), the
current spiraled to the right, forming a substantial east-
ward flow which is referred to as the mid-depth current.
At two stations, deep scuba dives were made to verify the
southern flow of the deep current previously documented
with the deep current drogues.

Figure 8 is constructed from all the deep-drogue mea-
surements and selected surface-drogue measurements
made during the summer and winter periods. These data
points represent end points of current vectors emanating
from the origin. The shaded spiral indicates the resulting
current structure. There arc not sufficient data to resolve
the spiral more accurately. The spiral reveals the basic
three-current system: the surface current (0 to 10 m) is
southwesterly, the mid-depth current (10 to 30 m) is
northeasterly, and the deep current (30 to 50 m) flows
southward.

The vertical current structure, as summarized by
Fig. 8, can be altered by the cross-reef currents and tidal
currents. The diagonal lines in Fig. 9 delineate the area of
the lagoon directly affected by the windward cross-reef
currents. At the northern end of the lagoon (region 1 in
Fig. 9) these currents follow the contour of the atoll. Along
the central part of the windward back-reef (region 2) the
current may be going north, west, or south, depending on
the tide and surf conditions. Near Enewetak Island (region
3) the current also follows the contour of the atoll. The
surface current and deep currents in regions 1 and 2 move
in the same direction when large volumes of water cascade
over the reef. During spring low tide, however, when little
water enters the lagoon over the reef, a surface current,
mid-depth current, and deep current characteristic of the
open lagoon can be observed (Fig. 7).

The currents directly behind the windward reef are
variable in speed, being fastest when large surf drives
water into these regions. Figure 10 is a plot on two suc-
cessive days, showing the current increase with rising tide.
Notice that the second date had higher surf and a slightly
higher wind speed than the first date. The two linear
regression coefficients are significantly different at the 95%
significance level. These data were taken at the site
denoted "A" in Fig. 9.

The cross-hatched areas in Fig. 9 delineate water that
experiences reversing current through the Deep Entrance
and the Southwest Passage. The area near the
northwestern leeward reef, marked by circles in Fig. 9, is
an area of convergence. The lagoon surface water cannot
escape over the leeward reef, particularly when large surf
drives oceanic water over these reefs into the lagoon.
Large aggregations of jellyfish have been observed in this
region, as well as strong southwesterly flow along the
lagoonward margin of the reef.

WATER BUDGET

Table 2 is a summary of the volume transports for the
important components of the water budget.

Input

The water can flow into the lagoon from the windward
reef, the Deep Entrance, and the Southwest Passage. The
windward cross-reef current transports about twice as
much water as the Deep Entrance current. Because the
windward cross-reef current never reverses, the volume
transport over the windward reef represents net input of
water into the lagoon. The Deep Entrance and the
Southwest Passage show net transports of approximately
zero over each tidal cycle.

Output

The water can flow out of the lagoon from the leeward
reef, the Deep Entrance, and the Southwest Passage.
Because the Deep Entrance and the Southwest Passage
have net transports near zero over each tidal cycle, the net
inflow from the windward reef must exit as outflow over
the leeward reef and out the South Channel. Because the
flow over the leeward reef is relatively small (Table 2),
most of the water flows south, exiting out of the South
Channel.

The numbers in Table 2 do not sum to zero over a
tidal cycle; however, these data were collected during dif-
ferent tide stages. Ranges were included in the table to
indicate the natural variability of the system.

CIRCULATION MODEL

Lagoon circulation can be explained as a response to
three sources of energy: (1) the surf on the windward
ocean reef, (2) the wind, and (3) the tides.

ATKINSON

Fig. 6 Photographs a and b show right-handed vertical current profile. Photograph c shows the left-handed vertical current pro-
file 2 km north of Enewetak Island. Direction of current and depth of dye dispenser are shown in the line drawing below the pho-
tographs. See Fig. 7 for location. [From Atkinson et al. with permission.]

Surf

The breaking waves on the windward reef drive water
over the windward reef flat and into the lagoon primarily
on the eastern (prevailing windward) side of the atoll. The
cross-reef currents and the currents behind the reef are,

therefore, dependent on the surf height and the depth of
water on the reef. This oceanic water spreads into the
lagoon, moving downwind and mixing vertically and hor-
izontally. Since the South Channel is the only significant
region of outflow, the water column has a net transport to
the south. This southerly net volume transport must

OCEANOGRAPHY

2.4 -^JT ■ 16

0 2 4 6

Fig. 7 Vertical current profiles. Number at end of arrow
gives depth in meters. Arrow gives direction of the current.
Circled number gives the depth of the bottom in meters.
[From Atkinson et al. with permission.]

TRADES

Surface
current
(0-10m)

Deep-water
current
(30-50m)

cm s-1

A 0-10m
A 10- 20m
O 20- 30m
• 30- 40m
■ 40- 50m

Fig. 8 Summary of drogue results. The shaded spiral
represents the approximate endpoints of current vectors from
the origin. [From Atkinson et al. with permission.]

Windward Reef

jy peep
;^;; Entrance

South Channel

Fig. 9 General current patterns in the lagoon. See text for
key.

increase toward the south end of the atoll to
acconnmodate the inflowing water across the windward
reef. Figure 11 shows the relative increase in southerly
volume transport versus distance from the north end of the
atoll. These relative volume transports are based on
cumulative transport across the windward reef. These are
reported as relative values, since variation in swell and
wind direction alter this relationship an unknown amount.

Wind

The wind creates the downwind drift of the surface
water and the upwind drift of the mid-depth water. These
can be qualitatively described as a special case of Ekman
wind-driven circulation. This pattern is superimposed on
the net drift of the entire water column toward the South
Channel. This southerly drift can be observed in the deep
water, below surface layers affected by the wind.

The northern end of the atoll has a relatively small
southerly drift based on cumulative net input (Fig. 11);
therefore, the effect of the wind can be observed at a
deeper depth in the north end than in the south end of the
atoll. Drogues suspended at 38 m in the north end moved
northeast to east, whereas drogues at a similar depth in
the southern end moved south. The increasing volume
transport from north to south, due to the increasing net
input over the windward reef, creates a southerly deep
current that thickens toward the southern end of the
lagoon. Figure 12 is a plot of the 38 m drogue directions
versus distance from the north end of the atoll. By the
middle of the lagoon, the layer at 38 m is well within the
southerly deep current (Fig. 12).

ATKINSON

w 10 -

E
o

270

1200

0.15

1300 1400 1500
Time of day (h]

1600 1700

1.5

Tide height (m)

Fig. 10 Change in current speed as a function of tide helgfit
near Runit on successive days. Average current speed plotted
at midpoint of time interval.

HIGH

3
O

L^ 1 I I .

10 20 30

DISTANCE (km)

Fig. 11 Increase in cumulative net input over the windward
reef flat as a function of distance from the north end of the
lagoon.

360 -

-o

090

180 -

270

10 20

DISTANCE (km;

Fig. 12 Change in direction of drogues suspended at 38 m
as a function of distance from the north end of the lagoon.

The observed pattern of wind-driven currents resembles
in many ways the pattern predicted by Ekman for an
enclosed sea in which the following conditions apply: (1)
impermeable, closed boundary; (2) constant, unidirectional
windstress over the entire surface; (3) homogeneous water;
(4) uniform depth; and (5) constant eddy viscosity. At
Enewetak these conditions are only partially met. The
lagoon rim is closed neither to leeward nor windward; in
particular, large quantities of water are introduced along
the windward edge (Table 2).

In a fully enclosed sea, the Ekman flow integrated over
depth is zero at every point. In a lagoon such as Enewetak
this will not be the case, but the detailed effects of the
"leaky" boundary and the irregular bathymetry have not
been estimated from present data. The remarkably shallow
spiral pattern of currents is a new finding which should be
further investigated and modeled.

Surface current speeds are 5 to 20 cm s~', approxi-
mately 2% of the wind speed. The surface drift is generally
downwind and seems responsive to the wind direction of
the previous 6 to 12 hours. The mid-depth upwind current
speeds are about one-half of the surface current speeds.

These wind-driven currents would cause the surface
water to overturn in 5 to 10 days if there was no vertical
mixing. Von Arx (1948) estimated approximately the same
time for turnover at Bikini. Von Arx (1949), Munk et al.
(1949), and Ford (1949) suggested that the surface water
at Bikini sinks in the western portion of the lagoon and
upwells in a small band in the eastern portion of the
lagoon. No direct evidence of upwelling has been found at
Enewetak. Upwelling, if it exists as such, will be largely
intermittent, because of the intermittent (tidal) pulsing of

OCEANOGRAPHY

the windward cross-reef inflow of surface water. At a max-
imum (high tide, active surf), this inflow is approximately
equal to the downwind transport of the lagoon surface
layer; during these intervals, upwelling is not required by
continuity to supply the wind-driven surface transport. The
essentially vertical homogenous water in Enewetak Lagoon
suggests that surface water mixes with bottom water
before reaching the leeward side. It also does not allow

any conclusions regarding the presence or absence of
upwellings from distributions of water properties. At Bikini,
Ford (1949) was able to follow the motion of discrete
water distinguished by salinity variations.

Surface, mid-depth, and deep water salinities at
Enewetak are shown in Figs. 13a, b, c. These salinity con-
tours show some of the general features of lagoon
circulation. Surface water was slightly less saline than deep

(b)

(c)

Fig. 13a, b, c Salinity for surface (0 to 10 m). mid-depth (10 to 30 m). and deep (30 to 50 m) water. (Collected by S. V.
Smith, July 26 to Aug. 16, 1974.)

ATKINSON

water because the weather was rainy during the collection
period. Relatively high salinity ocean water cascades over
the windward reefs and flows in through the Deep
Entrance.

Northeast trade winds blow less saline surface water
downwind with a buildup in the northwest region of the
lagoon. Because water is trapped in the leeward side of
the lagoon, return flow develops in the deeper water. The
low salinity return flow is mixed with surface water, creat-
ing a relatively vertically well-mixed water column with low
salinity downwind and high salinity upwind. During long
dry periods, opposite salinity gradients might be expected,
with high salinities downwind and relatively low salinities
upwind. Only a small portion of downwind surface water
escapes out of the Southwest Passage. The excess water
must move south toward the South Channel; consequently
isohalines bend toward the south (Fig. 13). There is no evi-
dence of a discrete water mass sinking on the downwind
side of the lagoon, flowing upwind as deep water, and
upwelling on the leeward side of the lagoon (as reported
by Ford, 1949 at Bikini). The water column appears
vertically well mixed (Table 1 and Fig. 13). There is also
no suggestion that water can maintain vertical structure for
5 to 10 days at Encwetak. As ocean water pours over the
windward reefs and into the lagoon, it mixes ver-
tically and horizontally as it moves downwind. Conse-
quently the salinity gradient is low to high, west to east,
regardless of depth. The water on the windward side of
the lagoon is predominately ocean water, but water on the
western side of the lagoon reflects net processes in the
lagoon. Scuba divers can observe strong mixing on the
upper vertical wall of the West Spit. Lower salinity lagoon
water mixes with high salinity ocean water in this region.
Phosphate and nitrate are lower in the western lagoon
water than in eastern water. Because water on the eastern
side in general reflects net lagoon processes, low nutrients
in that water suggest net uptake of these nutrients into the
ecosystem. Net organic production of benthic ecosystems
has been estimated by net uptake of nutrients (Smith and
Jokiel, 1976; Atkinson, 1981; Smith and Atkinson, 1983).
The observed decrease in these nutrients indicates a rea-
sonably low, net organic production for the atoll.

Ford suggested oceanic eddies might move through the
broad open channel at Bikini, the Enyu Channel. Perhaps
this process might occur in open lagoons; however, it docs
not appear to occur at Enewetak. Large eddies would be
destroyed when flowing into the lagoon by strong tidal
currents in the channels. Although a large eddy could not
be maintained, large oceanic eddies moving by the atoll
could influence the chemical and biological composition of
inflowing water.

Tide

Tidal currents directly influence the flow of water
within several kilometers of the passes, especially in the
southern part of the lagoon. These tidal currents can
overwhelm the wind-driven circulation, leading to such

local effects as the "left-hand" spiral observed two kilome-
ters north of Enewetak Island (Fig. 6c).

RESIDENCE TIMES

In the most elementary analysis, the average residence
time of water in the lagoon can be estimated by dividing
the lagoon volume by the net rate of water input. The cal-
culation yields a residence time of 33 days. Clearly there
is a variation of actual residence time from one part of the
lagoon to another because: (1) the water is introduced all
along the windward reef, but exists primarily through the
South Channel; and (2) there is no major north-south
recirculation mixing northern waters with southern water.
Thus, the residence time for water entering the north end
of the lagoon will be relatively long; water entering across
the southern reef will have a short residence time.

Because the water entering the northern lagoon must
transit the entire lagoon before exiting and because it
undergoes mixing by the superimposed wind-driven circula-
tion during that transit, a very simple estimate of the
residence time for that part of the inflow will have at least
qualitative validity. If it is estimated that the northern part
of the lagoon receives one-quarter of the total inflow, then
the residence time for this water (under the same very sim-
ple assumptions) will be four times longer than that for the
lagoon water as a whole, or 132 days.

Water entering the system in the north is of particular
interest because it flows across the areas with high
bottom-sediment concentrations of transuranic radionu-
clides (Nelson and Noshkin, 1972). Figure 14 is a general-
ized plot of sediment radionuclide activity; it indicates that
if release into the water column is proportional to the con-
centration in the sediment then most of the radionuclides

E
>-
>

DISTANCE (km)

Fig. 14 Decrease of sediment radionuclide activity as a
function of distance from the north end of the lagoon.
Radionuclides include "Sr, ^^u, '^Cs. "Co.

OCEANOGRAPHY

arc released into northern lagoon water, which has
residence times well above the average for the whole
lagoon. The concentrations of radionuclides in the water
column decrease from the northern end of the atoll to the
southern end, by a factor of 2 to 5 (Noshkin et al., 1974).
This horizontal gradient reflects the general increase in
flushing rate in the south end of the lagoon, as well as hor-
izontal diffusion from the north end.

The water column is vertically well mixed in terms of
temperature and salinity. However, in the central lagoon
the horizontal diffusion rates for certain materials may be
greater in the surface water than in deep water. Near the
windward reef, where both surface and deep currents
respond to the cross-reef currents, vertical transport may
be greater than in the central lagoon, and there may be no
difference in horizontal diffusion rates between surface
water and deep water.

VON ARX MODEL FOR BIKINI

Von Arx's (1948) model conceptualizes lagoon circula-
tion by linking two basic patterns: a "primary circulation"
and a "secondary circulation."

The primary circulation consists of wind-driven surface
water moving downwind, sinking, and then returning
upwind to the windward (eastern) side of the atoll lagoon
as deep water.

The secondary circulation consists of horizontal recircu-
lation of deep water. Von Arx reported that the volume
transport of the eastern flowing deep current is greater
than the volume transport of the surface current. He con-
cluded that some of the deep water is shoaled upward or
"upwelled" in the eastern part of the lagoon, becoming the
surface current. The remaining portion of the deep water
diverges at the leeward edge of the windward reef. Some
water moves northward following the bathymetric contour
of the basin. The deep water circulation forms two
counter-rotating bodies of water, the northern one moving
counterclockwise and the southern one moving clockwise.

Von Arx estimated that the exchange of lagoon water
through all channels and passes during winter is approxi-
mately 3.8% of the total lagoon volume per tidal cycle. At
a 30% exchange efficiency, von Arx estimated the winter
Bikini lagoon flushing to be 39 days. The summer flushing
time was estimated to be twice as long as that in the
winter.

The conspicuous feature of von Arx's model for deep
atoll lagoon circulation is the deep return flow toward the
windward side of the atoll. This return flow connects the
primary circulation with the secondary circulation. The
model for the circulation system of Enewetak has some
similarities to the model proposed by von Arx for Bikini.
The primary circulation system consisting of an overturning
wind-driven surface current is the same in terms of speed
and volume transjxjrt.

The secondary system, or deep circulation, is not the
same as that proposed by von Arx. The deep current at

Enewetak flows southward, toward the channel having net
outflow. Von Arx described a horizontally recirculating
deep current with a volume transport greater than the sur-
face current, hence upwelling on the windward side. At
Bikini the large open channel (Enyu Channel) is at the
southeastern end of the lagoon. A net transport toward
this channel would create an eastward flowing deep
current. The eastward mid-depth current and the "pass-
ward" deep current would then appear to be a single deep
current with a mass transport greater than the surface
current. The excess volume transport of von Arx's deep
current might largely be balanced by net outflow through
Bikini's southeastern channel. Von Arx did not report a
large net outflow; however, recalculation of his data sug-
gests net outflow through the Enyu Channel. Outflow was
also shown in the distribution of indigenous zooplankton
(Johnson, 1949) and was observed in surface radionuclide
patterns (Noshkin et al., 1974). To reach the Enyu Chan-
nel, the deep water in Bikini Lagoon must move east-
wards. In Enewetak Lagoon the only effective exit is at the
southernmost part of the atoll; therefore, the deep water
must move southward.

Note that in the model derived from Enewetak, the
deep motion is primarily controlled by the location of the
major exit points from the lagoon. Water flow through
other atoll lagoons seems to be regulated by atoll morphol-
ogy and local wave and tidal conditions (Milliman, 1967;
Gallagher et al., 1971; Henderson et al., 1978; Ludington,
1979). Studies of deep currents in other deep lagoons
could be valuable in testing this interpretation.

CONCLUSION

Windward and leeward cross-reef currents, channel
currents, and tidal flow are the major factors influencing
the exchange of water between atoll lagoons and the sur-
rounding ocean. Because these factors are specific to local
wave climate, tidal conditions, and atoll morphology, atoll
lagoons have widely varying flush characteristics. Wind-
driven circulation, a pervasive feature of lagoons, con-
tributes primarily to internal circulation rather than flush-
ing. Upwelling on the windward side of lagoons may occur
as a summation of the above phenomena but does not
seem to be a generalizable feature of deep lagoon circula-
tion. Deep water flow appears to orient itself toward the
channels of net water output.

ACKNOWLEDGMENTS

This chapter is based on the final report of EXDE con-
tract number EY-77-5-08-1529, Water CiTculation of
Enewetak Atoll Lagoon and Circulation of Enewetak Atoll
Lagoon, by M. J. Atkinson, S. V. Smith, and E. D. Stroup.
Parts of the research were done under the auspices of the
Mid Pacific Research Laboratory. Thanks to S. V. Smith
for chemical data and review of the manuscript.

ATKINSON

REFERENCES

Atkinson, M J., 1982, Phosphate Flux as a Measure of the Net

Coral Reef Productivity, in Proceedings of the Fourth Interna-
tional Coral Reef Symposium. Manila, 1: 412-418.
S. V Smith, and E D Stroup, 1979, Water Circulation of

Enewetak Atoll Lagoon, Final Report, DOE Contract EY-

77-5-08-1529.
S. V. Smith, and E. D. Stroup, 1981, Circulation in

Enewetak Atoll Lagoon, Limnol. Oceanogr , 26: 1074-1083.
Barnes, C A., D. F Burmpus, and J Lyman, 1948, Ocean Cir

culation in the Marshall Islands Area, Trans Am Geophys.

Union. 29: 871-876.
Buddemeier, R. W., 1982, The Geohydrology of Enewetak Atoll

Islands and Reefs, in Proceedings of the Fourth International

Coral Reef Symposium, Manila, 1: 339-345.
Ekman, V W , 1905, On the Influence of the Earth's Rotation on

Ocean Currents, Ark Mat, Astron. Fysik, 2: 1-53.
Emery, K. O., J. 1 Tracey, Jr , and H. S. Ladd, 1954, Geology

of Bikini and Nearby Atolls, U S Geol Sum. Prof. Pap ,

260-A. pp. 1-265.
Ford, W L , 1949, Radiological and Salinity Relationships in the

Water at Bikini Atoll, Trans, Am, Geophys Union. 30: 46-54
Gallagher, B S., K. M. Shimada, F. I. Gonzalez, Jr., and E. D.

Stroup, 1971, Tides and Currents in Fanning Atoll Lagoon,

Pac. Sci., 25: 191-205
Hamner, W. M , and 1 R. Hauri, 1981, Effects of Island Mass:

Water Flow and Plankton Pattern Around a Reef in the Great

Barrier Reef Lagoon, Australia, Limnol. Oceanogr . 26:
1084-1102.
Henderson, R. S., P. L. Jokiel, S. V. Smith, and J. G. Grovhoug,
1978, Canton Atoll Lagoon Physiography and General

Oceanographic Observations, Atoll Res. Bull., 221: 514.

Johnson, M W., 1949, Zooplankton as an Index of Water

Exchange Between Bikini Lagoon and the Open Sea, Trans,

Am. Geophvs Union, 30: 238-244.
Ludington, C. A.. 1979, Tidal Modifications and Associated

Circulation in a Platform Reef Lagoon, Aust J Mar,

Freshwater Res., 30: 425-430.
Milliman, J P., 1967, Carbonate Sedimentation in Hogsty Reef, a

Bahamian Atoll, J Sediment. Petrol., 37: 658-676.
Montgomery, R B , and E. D Stroup, 1962, Equatorial Waters

and Currents at 150°W in July-August, 1952, Johns Hopkins

Oceanogr Stud , 1: 1-205.
Munk, W H , G C. Ewing, and R. R. Revelle, 1949, Diffusion in

Bikini Lagoon, Trans. Am. Geophvs. Union, 29: 59-66.
and M. C. Sargent, 1949, Adjustment of Bikini Atoll to

Ocean Waves, Trans. Am, Geop/iys. Union, 29: 855-860

(also U. S. Geol. Surv Prof Pap 260 C).
Nelson, V., and V. E. Noshkin, 1973, Enewetak Radiological Sur

uey. U. S Atomic Energy Commission, NVO-140V, pp

131 225
Noshkin, V E, K M Wong, R. J. Eagle, and C Gatrovsis,

1974, Transuranjcs at Pacific Atolls, 1, Concentrations in the

Wafers at Enewetak and Bikini, University of California, Liver-
more Rep , 51612: 1 30
Smith, S. v., and P L Jokiel, 1978, Water Composition and

Biogeochemical Gradients in Canton Atoll Lagoon, Atoll Res,

Bull , 221: 15-53.
, and M. J. Atkinson, Mass Balance of Carbon and Phos-
phorus in Shark Bay, Western Australia, Limnol. Oceanogr ,

28(4): 625-639.
Von Arx, W W., 1948, The Circulation Systems of Bikini and

Rongelap Lagoons, Trans. Am. Geoph^/s. Union, 29:

861-870
, 1954, The Circulation Systems of Bikini and Rongelap

Lagoons, U S. Geol Suru. Prof Pap. 260-B, pp. 265-273.

Chapter 6

Meteorologi; and Atmospheric Chemistry;
of Enewetak Atoll

JOHN T. MERRILL and ROBERT A. DUCE

Center for Atmospheric Chemistr\/ Studies

Graduate School of Oceanography^

University^ of Rhode Island, Kingston, Rhode Island 02881

INTRODUCTION

The Marshall Islands area has a marine climate that
varies from tropical to subtropical; near Enewetak Atoll
the weather is characterized by brisk steady winds,
moderate rainfall, and unvarying high temperatures with
typical partial cloudiness. The atoll lies near the northern
edge of the tropical zone dominated by the migrating equa-
torial trough of low pressure, with its heavy rains. It lies
well within the northeast trade wind area of the North
Pacific; that is, the surface winds are from the east and
northeast on average. There have been more than 20
years of careful meteorological observations at the airstrip
on Enewetak Island, and we make use of some of the
archived data to discuss, in turn, the various aspects of the
weather. In the section on climate and weather, we cover
briefly the mean and variation for each observed quantity
of interest and note our state of knowledge of these fac-
tors. Also in that section we set out an annotated bibliog-
raphy of sources of additional data and of specialized dis-
cussions. In the section on the atmospheric chemistry of
the atoll, we make use of the extensive data collected dur-
ing experiments there in 1979.

We discuss both the mean value and exfjected range of
variation because neither alone covers all of the weather.
The variability of the weather is the combined effect of dis-
turbances of various scales which may have well-defined
structures in space and time and of phenomena that can
be taken as random. We begin by discussing some of the
more common structured disturbances. Over the years
diurnal variations at island sites have been discussed and
analyzed. While there is no doubt that there are diurnal
cycles in cloudiness and precipitation, no attempt is made
here to provide explanations for them in terms of first

causes because the interaction can be both subtle and com-
plex. Also, at short periods there is the atmospheric tide,
primarily a thermally driven effect that produces global
pressure fluctuations and wind patterns that are rather
complex. The influence of the atmospheric tide at the sur-
face, though greatest in the tropics, is relatively small, and
we mention it only briefly. The semidiurnal fluctuation is
the dominant mode of the tide and has an amplitude of
about 1 mbar, or 85% of the diurnal variance about the
annual mean of 1010 mbar pressure at Enewetak. Chap-
man and Lindzen (1970) developed the presently accepted
theory of the tide. The discussion by Lavoie (1963) of cal-
culations available at that time is superseded, despite the
absence of seasonal effects in Chapman and Lindzen's
basic model. Nevertheless, the data presented by Lavoie
for the monthly variation of tide parameters are correct
and illustrative, despite the relatively short record.

Disturbances lasting a day or more are common in the
tropics, and we discuss them in the sections entitled "Trop-
ical Storms and Disturbances" and "Winds Aloft." There
are two seasons at Enewetak, the dry season from
December through March and the wet season from April
through November. Annual variation is crucial to under-
standing the weather in the tropical marine environment,
and this influence is included in each section, particularly
in the section on precipitation.

Although there has been much work recently on the
variability of climate over periods of a year to a decade,
we cannot say much yet about how such changes affect
the tropical islands. It is known that there are quasi-
periodic fluctuations in the strength of the Pacific trade
winds correlated with equatorial sea surface temperature
variations at very large scales and that there follows a
chain of consequences that includes changes in both tropi-
cal and mid-latitude circulations. A clear exposition on this
subject, the Southern Oscillation, is to be found in Tren-
berth (1976). While much of the present interest stems
from the possibility that disturbances in mid-latitude
weather and coastal upwelling could be forecast months
ahead, we will certainly learn much about the tropical cli-
mate itself from the numerous studies now under way.

MERRILL AND DUCE

CLIMATE AND WEATHER OF
ENEWETAK ATOLL

Temperature and Humidity

It is obvious that high surface temperature and hu-
midity are to be expected on tropical islands. It is less
obvious, but well documented, that it is difficult to obtain
accurate temperature measurements in an op)erational pro-
gram in such environments because of such factors as radi-
ational heating of the shelter in daylight. Thus it is likely
that the air temperature range rep>orted below is exag-
gerated by about 0.5°C (see Lavoie, 1963, for a discus-
sion). This is a small enough error for temperature, but it
significantly degrades the accuracy of the relative humidity.
Nevertheless, we can see that there is relatively little
change in these quantities through the year and that a reg-
ular diurnal cycle is evident. The temperature and hu-
midity both respond noticeably and regularly to rain
showers, but in the data presented here the nearly random
occurrence of rain with time has smoothed out this effect;
in fact, even hourly data do not show the full effect of
short-lived rain events.

The temperature and humidity data shown in Fig. la-c
are from the U. S. Air Force measurements now archived
by the National Climatic Center. Made at hourly intervals
between 1945 and 1969 (with irregular breaks), the obser-
vations correspond to 14.1 years of uninterrupted mea-
surement. This record is sufficiently long that the overall
pattern and its variability can be perceived. The data aver-
aged over 3-hour periods are displayed as a function of the
hour and of the month; the draft plot is extended beyond
the borders shown so that edge effects are minimized.

That the temperature depends very little upon the time
of year can be seen in the mostly horizontal contours
shown in Fig. la. Also note that the highest daily tempera-
ture is recorded between 12 and 15 hours local standard
time and lies between 28.5 and 30°C; the lower
temperature is observed in the dry season and the higher
during the wet season. In the morning and in the evening,
the temperature depends even less upon the time of year,
with values increasing and decreasing daily through the
upper 20s. In the hours after midnight, the decrease of
temperature slows in the dry season and ceases in the wet
season, with the lowest average value reaching 26 to
27°C.

Now these are monthly and 3-hour averages over years
of data, and even though these patterns are generally
valid, there are fluctuations. The representativeness of this
pattern can be seen in the small variances: just over 0.5°C
at night to a maximum of under 1.5°C in the afternoon in
October. (These are variances of hourly data averaged
over 3-hour periods for each month, i.e., variances about
the mean shown in Fig. la.) This method of averaging
does not accurately record the average maximum and
average mmimum hourly temperatures for each day. These
are given in Table 1 for each month of the year. The
range of temperature is greater here, as expected. The

average minimum temperature for each month is nearly
independent of month at about 23°C, whereas the average
maximum exceeds 32°C in August and September and is
30°C during the dry season.

(c)

M J J A

MONTH

Fig. 1 Temperature and humidity data for Enewetak. (a)
Dry bulb temperature, °C; (b) Relative humidity in percent: (c)
Dew point tempicrature, °C. These are three hour averages for
each month. Contour interval is 0.5°C for temperatures. 2.5%
for humidity.

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

TABLE 1
Average Minimum and Maximum Temperatures, °C

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Minimum 23 5 23 4 23 6 23.8 23.5 24.0 23.6 23 6 23 8 23 4 23 7 23.8
Maximum 30.4 30 5 30.6 31.1 31.4 31.6 318 32.3 32.3 32.1 31.7 30.9

Extremes of temperature have also been recorded but
provide little additional information. The annual minimum
value of 21°C has occurred once in the record and the
maximum of 34.4°C three times. The values reported by
Blumenstock and Rex (1960) are less reliable because of
shorter records. As mentioned above, it is possible that the
maximum temperatures are overestimated.

Similarly averaged relative humidity data shown in Fig.
lb exhibit a bit more time dependence than the tempera-
ture. There is a broad maximum in the early morning fol-
lowed by decreasing values as the temp)erature rises in
daylight. The afternoon minimum is lower but briefer in
the dry season than it is in the wet season. These values
are the best estimates available but may be underestimated
during the day by several percent if the temperature is
overestimated by 0.5°C. The dew point temperature — the
temperature at which saturation will occur if the air is
cooled — is less sensitive to such error and is a straightfor-
ward indicator of the moisture content of the air. The dis-
tribution shown in Fig. Ic seems quite different from, but
is entirely consistent with, the data in Fig. la and b. The
dew p)oint temp)eraturc is strongly dependent upon the
time of year, with a broad minimum in the dry season and
a broad maximum in the wet season. In both cases there is
an increase during the daylight hours. This increase
represents a slight increase in the water-vapor mixing ratio,
which is consistent with increased evaporation during the
day. It is this increase which prevents the relative humidity
from decreasing more than it does in the afternoon.

Precipitation

Across most of the North Pacific, including the
Marshall Islands area, the rainfall increases markedly from
mid-latitudes to just north of the Equator. At Wake (19°N)
the annual total is 940 mm, at Kwajalein (9°N) about
2400 mm, and at Jaluit (6°N) it exceeds 4000 mm. The
highest values are in the equatorial trough near 3° to 6°N
and the lowest in the subtropical high pressure area at
about 25°N, well east of the dateline. Enewetak at 11°N
lies near the northern edge of the zone of most rapid
decrease of rainfall with latitude. The average rainfall of
1470 mm is not distributed uniformly through the year;
about 85% comes during the wet season, which starts in
April and ends in mid-November. The variability of the
rainfall is remarkable, and this factor is a central theme in
the discussion which follows.

There is much contention in the meteorological litera-
ture over the applicability of island-based rainfall data to
open ocean conditions. This is not an issue here as we are
concerned with the rainfall at the atoll, not in the environ-
ment in its absence. We note briefly below the limited data
available to discern gradients across the atoll.

The rainfall distribution through the year, which is
based on the archived data tabulated by Taylor (1973), is
shown in Fig. 2a. The three measures shown for each
month are the rainfall amount exceeded in 90, 50, and
10% of the years. Thus the amount expected (50%
occurrence) in November, about 124 mm, is somewhat
less than the 140 ram in the "rainiest December in 10
years." It can be seen at once that in certain months there
is a very large range of variability. Although the average
April has about 40 mm of rainfall, one year in ten may
have less than 10 mm; another may have over 260 mm.
The record represented here is 32 years long; there are
missing data, but 26 to 29 years of monthly values are
available in the various months. A longer series would
probably not change the annual total much, and because it
is inherent in tropical rainfall, the fluctuation evident in
Fig. 2a would not be reduced by additional years of obser-
vation.

For this discussion, we let the dry season begin with
December. That this is arbitrary can be seen by comparing
the 10% value in Fig. 2a for December with the 50%
value for November and the 90% value for November with
the 10% value for December. When the wet season ends
early, the November rainfall is less than 120 mm, whereas
when it ends late, the December total exceeds 50 mm.
The January, February, and March 50% values all lie
between 20 and 35 mm, and the 90% values are uni-
formly very small at <10 mm. Many of the dry season rain
events are from small cumulus clouds; however, these
affect the total amount less than the infrequent distur-
bances. We follow the usual designation and let the wet
season begin in April despite the small increase in the 50%
value; this may be rationalized by the jump in the 10%
figure — i.e., some Aprils are very wet.

The increased rainfalls of May and June are followed
by 50% values between 175 and 225 mm in July through
September. The maximum is in October, which also has
the highest average and the greatest 90% total. November
is a transitional month. The number of days with measur-
able (>0.25 mm) rainfall is greatest in August at 21 on the
average; this figure varies between 10 and 21 for the wet
season months, while it is 10 to 15 during the dry season.

MERRILL AND DUCE

WET SEASON
DRY SEASON

(b)

RAINFALL DEVIATION

Fig. 2 Rainfall data for Enewetak. (a) Rainfall amounts for
each month. The three measures correspond to the amount,
in mm, exceeded in 90% (below shading). 50% (above) and
10% (top) of the months in the record, (b) Deviation frequen-
cies for the wet and dry seasons. The curve shows the percent
occurrence of deviations from the seasonal mean in units of
a. the square root of the variance. The corresponding rain
amounts, in mm are shown below the axis.

Thus, as noted above, there are many small rain showers
even in the dry season. The wet season amount includes
such cumulus showers and a greater number of larger, sus-
tained rainfalls during disturbances and during the infre-
quent tropical storm or approach of the equatorial trough;
these are much less common in the dry season.

Another way to look at the variability of the precipita-
tion is shown in Fig. 2b, where the frequency of
occurrence of monthly amounts is given as a function of
the deviation from the season average. Note first that in
both regimes the most common value is significantly lower
than the mean, about 0.5 a below. (Here a is the root
mean square deviation of rainfall amounts for the season.)

In the dry season, very dry months are common, but a few
months with large amounts of rainfall do occur. For exam-
ple, 2% of the years would be expected to have a "dry"
month with 160 mm of rain, 2.5 a above the average.
Note also that the overall occurrence of the large rainfall
months is not dependent upon the season but that the
amount of precipitation is. The infrequent 3 or 4 a cases
for the wet season corresfsond to very substantial totals.
These frequency distributions are typical of subtropical
sites but are somewhat uncertain far from the mean value
because of the limited length of record. Also, since these
frequencies were averaged over the entire season, the
month-to-month variation, which is large in the wet season,
is lumped together with the interseasonal difference here.

Insufficiency of data limits one's ability to document
diurnal variation In precipitation amount. Nevertheless,
Lavoie (1963), using primarily Enewetak data, presented
convincing evidence of an early morning maximum in
frequency of rain; the deviation is fjerhaps 15% at the
peak. There is some evidence in the same data set for a
broad and weak afternoon minimum in the rain
occurrence. Lavoie considered several mechanisms in an
attempt to rationalize these values and to explain some sig-
nificant difficulties: the data have large scatter, and even
the maximum does not appear at every station. It thus
seems best to say that there is a tendency for a maximum
in the rainfall occurrence in the early morning and a weak
minimum in the afternoon.

Even more limited are data giving the spatial distri-
bution of rain about the atoll. Any variation is assumed to
be primarily random because of the low relief of the
atoll — i.e., the absence of orographic forcing. However,
there could be sufficient disruption of the thermodynamic
structure of the atmosphere by the presence of the lagoon
to cause a discernable pattern. Data laboriously collected
by Blumenstock and Rex (1960) for six special sites on
islands around the atoll during 2-week periods, once in
each season, have not to our knowledge been carefully
analyzed in the literature. They reveal no systematic pat-
tern of variation. The rainfall amounts at the various sta-
tions are highly correlated only when the stations are close
together, and there is always some difference among them.
The record thus appears consistent with rain areas of vari-
ous sizes unforced by the atoll itself. Nevertheless, this
does not rule out some such forcing in other cir-
cumstances. This record does not include any disturbed
weather periods during which there could be a measurable
difference of rainfall across the atoll.

Cloud Cover and Solar Radiation

Accurate estimates of cloud distribution and type are
not easy to obtain, particularly at night and when low
clouds obscure the sky. As discussed by Blumenstock and
Rex (1960), there is likely a systematic bias —
overestimation. Fortunately the overall cloud amount is
least affected, and voluminous data exist for this quantity
in the archive. Again, the average variation with time of
day and time of the year is the main topic of discussion.

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

The fraction of the sky covered by clouds exceeded 75,
50, and 25% of the time as shown in Fig. 3, which is
based on the tabulated three hourly frequencies of cloud
cover in tenths.

The cloudiness is more variable in the dry season than
the wet; indeed, the daytime sky is covered Vio or more
75% of the time in July through October. More than 7,0
cover is common (^25%) in April through November,
and this frequency does not depend u|x>n the time of day.
The expected cloudiness (i.e., the 50% value) varies from
about Yio, higher in daylight and lower at night, in the dry
season to about '/lo, with somewhat less diurnal variation
in the wet season. The sky is seldom clear, even in the dry
season. Two-tenths of the sky is covered more than 75%
of the time (although this may be biased at night).

A different measure of cloud cover is obtained from
satellite observations. Images obtained routinely from geo-
stationary platforms show the aerial extent and temporal
evolution of cloud systems. In addition, radiometric mea-
surements of the cloud top temperature yield good esti-
mates of the cloud top height. Accurate measurements
below obscuring layers of high clouds are not yet obtain-
able routinely.

We are not aware of any published solar radiation data
for the Marshall Islands area, but there are data for certain
times in 1977 to 1979 (see "Sources of Additional Data").
It is obvious that the typical partial cloudiness and the high
moisture content of the near-surface air significantly dimin-
ish the incident sunlight. Working against this, however. Is
the long (and unvarying) day. The interval between sunrise
and sunset varies from 11 hours 29 minutes to 12 hours
46 minutes. The available data show these effects, with
the average value exceeding 21 X 10^ j m~^ d~' in

z J*^' f'EB MAR APR MAY JUN

2 ° 10 10 10 10 10

•" '■ " I'M T I I, I

01 ■ '

04
_l 07

5 '°

O 13

—I

AUG

SEP

OCT

NOV

DEC

Fig. 3 Cloud cover data for Enewetak. For each month the
three measures are the fraction of the sky obscured, in tenths,
at least 75% (left), 50%, and 25% (right) of the time for each
three hour period of the day.

many months; this value corresponds to 500 cal cm~^
d , a typical maximum total at mid-latitudes. Neverthe-
less, during disturbances the flux can be reduced for
periods of several days, and the value can drop below V^
of this figure for a day or two at a time.

Surface Wind

The surface wind data are shown in Fig. 4 as wind
roses for each month (a) and for the entire year (b). As
indicated in the key in Fig. 4b, in each rose the bar indi-
cates the frequency of winds coming from that direction
for each range of speed above calm. The numerical values
beside each bar are the frequency, in percent, for wind
from that direction and for that range of speed. The fre-
quency of calms, to which no direction is assigned, is
shown in the center of the circle. The frequency of
occurrence of wind in each range of speed for all directions
is shown, in percent, in the line below each rose. The wind

JAN

1 1

Fig. 4 Surface wind data for Enewetak. Wind roses for each
month (a) and for the year (b). Frequency of wind for each
directon and range of speed is shown by the printed figure
beside the bar. which shows the frequency of winds for that
direction for all speeds above calm. Frequency of calms is
shown in the center of the circle. Frequency of wind speeds
for all directions shown below each rose.

(Fig. 4a cont'd on next page)

MERRILL AND DUCE

FEB

APR

0 I 1 I e I 42 ! 37 I 12 I X I 0 I I X I .1 I 7 I 46 I 39 1 7 I X 1 0

MAR

MAY

I X I 1 I 8 I 46 1 36 I 10 I X I 0 I | 1 | 2 I 1 2 I 49 | 3 1 | 6 1 X ; 0 j

Fig. 4a cont'd. (Fig. 4a cont'd on next page)

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

JUN

AUG

1 1 4 ! 16 I 53 I 24 [ 3 ! X ' 0

4 I 15 I 31 I 40 7 1 X 0

JUL

SEP

2 I 9 I 28 I 48 i 11 1 1 I X : 0 I | 6 | 18 | 34 | 33 | 7 | 1 | X | X |

Fig. 4a cont'd. (Fig. 4a cont'd on next page)

MERRILL AND DUCE

OCT

NOV

5 I 17 I 31 I 35 I 7 I 1 I 0 I 0

1 I 5 I 20 I 44 I 23 I 6 I X I X

DEC

X I 3 I 1 1 I 42 I 33 I 9 I 1 I X
Fig. 4a cont'd.

(Fig. 4 cont'd on next page)

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

ANNUAL

Lll.

speed increments are indicated in the figure and are the
traditional Beaufort scale values. The data are from U. S.
Air Force records, as are the temperature and humidity
data above, and were collected at various intervals, hourly
over most of the period, with the instrument mounted at a
height of 40 feet above sea level. The data are from the
years 1945 to 1969 and again correspond in number to
14.1 years of continuous hourly observation. Thus the
representativeness of these figures is good and falls in the
range that one would expect from a sample of 10 to 20
years of continuous measurement. We have smoothed the
data to report them at eight compass points but were care-
ful to use a weighted averaging that preserves the rapid
falloff of wind occurrence away from the predominant east
and northeast directions.

This much-noted constancy of the wind is the first
aspect of the rose data that we examine. During much of
the year, the wind is from the northeast or east 95% or
more of the time. From July through October, however,
the peak broadens somewhat and moves a bit toward the
south so that less than 20% of the winds are out of the
northeast. The maximum frequency of winds from the less
common directions (southeast around to north) is in
August, September, and October, when disturbances are
most common and when the equatorial trough is closest on

12 3 4 5 6 7 8

1-3 4-7 8-12 13-18 19-24 25-31 32-38 39-46

1-3 4-6 7-10 11-16 17-21 22-21 28-33 34-40

03-1.6 2-3 3.6-52 5.7-83 8^-11 11-14 14-17 18-21

FREQUENCY OF CALMS: PERCENTAGE
IS SHOWN IN CENTER OF CIRCLE.

DIRECTION FREQUENCY; BARS SHOW
PERCENTAGE FROM EACH DIRECTION.
EACH CIRCLE EQUALS 10%.
20% OF ALL WINDS FROM NE.

SPEED FREQUENCY: FIGURES SHOW
PERCENTAGE FROM EACH DIRECTION IN
EACH SPEED RANGE. 6% OF WINDS
WERE FROM BETWEEN 13 AND 18 mph.

TABLE: FREQUENCY OF WIND FROM ALL
DIRECTIONS FOR EACH SPEED RANGE.

SPEEDS ARE THE BEAUFORT SCALE:

MILES / HOUR
NAUTICAL MILES / HOUR
METERS / SEC

Fig. 4b

MERRILL AND DUCE

the average. The highest frequency of brisk winds is in the
dry season, with over 45% of the hours having winds
>8.5 ms~' (19 mi h~'). During the wet season the wind
weakens substantially, particularly during August through
October when >50% of the hours have wind <5.4 ms~'
(12 mi h~^). Only during July through October are calms
at all common, i.e., greater than 1% occurrence.

The dry season months exhibit the greatest constancy
of pattern: >50% from the east and >40% from the
northeast, with >75% frequency of speeds between 5.8
ms"' and 10.7 ms"' (13 and 24 mi h^'). April does not
differ much, except that the strength of the wind decreases
slightly. In May and June the winds are strong out of the
east, while in July through October the speed decreases
and the direction varies more. In November the wind
begins to shift back to the dry season pattern.

As these are average winds, the pattern of variation
with time is lost. There is a consistent shift in the wind
associated with easterly waves, the most common distur-
bance type in the wet season. The correlation of wind
shifts with cloudiness and rainfall, obvious to anyone
present during such events, is lost.

The annual average wind rose shown in Fig. 4 is easily
understood given the monthly distributions discussed previ-
ously. Note that % of the time the wind is from 5.8 to
10.4 m/s (13 to 24 mi h '), and over 60% of the time
the wind is from the east. Nevertheless, the annual aver-
age shows at least 0.1% winds from every direction.

Tropical Storms and Disturbances

While tropical storms strike the Marshall Islands infre-
quently, disturbances in the weather are a common and,
on occasion, regular occurrence. Tropical storms of the
greatest strength are called typhoons in the western
Pacific, and they are, of course, extremely dangerous and
destructive, particularly to exposed areas at low elevation
such as Enewetak Atoll. Such storms grow from and are in
fact the most fully developed form of tropical disturbance.
We discuss the disturbances first because they are more
numerous.

Several types of tropical disturbances are recognized in
the literature; nevertheless, it is often impossible to classify
a given weather system as one of the several types, even
given estimates of the thermal structure and the movement
and growth of the system. We are concerned primarily
with the surface manifestation, so we shall only summarize
what is known about the most common disturbance types.

During the wet season, particularly July through Sep-
tember, westward propagating wave-like systems are com-
mon in the tropics and have been observed and analyzed
in the western Pacific and in the Caribbean and North
Atlantic Ocean areas. In the western Pacific these easterly
waves, on average, have a horizontal scale of 3500 to
4000 km and travel toward the west an average of 7°
longitude per day (i.e., a mean velocity of 9 ms or 20
mi h^'); thus the disturbance affects a station for 4.5 to
5 days. During the passage of such a wave, there is a

more or less systematic variation in the wind, cloud cover,
and rainfall. The north-south component of the wind
shifts, with maximum winds from the south of 1 to 2
ms"^ (2 to 5 mi h~') leading and maximum winds from
the north following the center of the disturbance. The max-
imum cloudiness and rainfall occur just after the pjassage of
the center of the disturbance. There is a temperature fluc-
tuation, but it is hardly discernable at the surface. These
waves can be observed with satellite images and are now
understood to be an inherent prop>erty of deep easterly
flow. The structure and detailed dynamic characteristics of
such waves in the Marshall Islands area were studied by
Reed and Recker (1971) using radiosonde and satellite
data. The waves are most common in the wet season
because the upper level winds are most favorable for their
growth then. About Vs of such waves increase in intensity
sufficiently to become classified as depressions or storms,
but this occurs most commonly well west of the Marshall
Islands.

Other types of disturbances are more uniformly dis-
tributed through the year but are even less easily classi-
fied. One type, the upp)er level cold-core low, is similar to
the subtropical cyclone that is often observed in the
Hawaiian area. In the Marshall Islands area, it may have
no surface manifestation or may be accompanied by a
weak but long-lived period of disturbed weather. In addi-
tion, there arc squall lines and other short duration events
which may produce strong winds and intense rainfall over
limited areas as they pass.

Although both the frequency and the destructive power
of tropical storms are greater in the far western Pacific
than in the Marshall Islands area, such storms can threaten
any tropical location. A sense of the seasonal distribution
and the range of impact possible can be obtained from
Table 2, which summarizes the depressions and storms
that affected Enewetak between 1959 and 1979. Of
course, the highest overall probability of tropical storm for-
mation in the area is during the wet season, particularly
July to October. However, there have been strong storms
well within the dry season (e.g., Alice in 1979). The high
winds and waves that extend to the periphery of such
storms can have devastating consequences. There is a sub-
stantial body of literature on the effects of such storms on
atolls, but the closest atoll so studied is Jaluit
(Blumenstock, 1961). Specific data about individual storms
are often sketchy, and prior to the operational use of satel-
lite images, the tracking of past storms when far from land
or shipping lanes may have been substantially in error.
Nevertheless, there are useful data on several storms over
the years, as indicated in Table 2.

Winds Aloft

The structure of the wind field above Enewetak Atoll is
complex and variable. At time scales longer than 2 years,
there are nearly periodic fluctuations at some levels, while
at other levels there are short-period variations as impor-
tant as those in mid-latitudes. In the following discussion.

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

TABLE 2

Tropical Storms and Disturbances
Affecting Enewetak. 1959-1979

Name,

Year

dates (GMT)

Remarl<s

1979

Alice,

Passed Kwajalein Jan 3, was of

Jan. 5-6

typhoon strength at Enewetak.
CD,-p 91

1976

Nancy,

Strengthened from a depression to

April 24-25

storm status as it crossed Enewetak;
later became a typhoon.
CD,' p. 102.

1976

Therese,

Passed from SE to SW as a distur-

July 7-8

bance; later became a minimal
typhoon. CD," p. 100.

1972

Kathy,

Tropical depression at Enewetak, later

Oct. 28

a storm CD,* p. 101.

1972

Olga,

Tropical storm at Enewetak,

Oct 24

strengthened later to typhoon
intensity. CD,' p 100.

1971

Faye,

Disturbance and depression at Enewe-

Oct 3

tak, later a tropical storm.
CD," p. 778.

1969

Phyllis,
Jan. 18

Tropical storm. CD," p. 78.

1967

Harriet,

Depression 100 miles south, later

Nov. 17

a typhoon. CD,' p. 76.

'CD, Climatological Data. National Summary, a NOAA publi-
cation from the Environmental Data Service, National Climatic
Center, Asheville, N. C.

we emphasize the most important aspects of the upper
level wind structure, proceeding upward from the surface
and covering each 3-month period and the east-west and
north-south components of the wind. The discussion Is
based on radiosonde data compiled and analyzed by
Newell et al. (1972) and, to a lesser extent, on the illus-
tration (their Fig. 4) of Blumenstock and Rex (1960).

The near-surface trade winds are strongest in the dry
season, and they extend at least some 5 km or so into the
atmosphere all the year around. The east-west component
of the wind is negative, i.e., the wind is from the east, up
to 500 mbar (~5.6 km) in March to May, and up to about
300 mbar (~9.5 km) in September to November. The
westerly winds which overlie this layer are absent, in the
long-term average, in December to February. The
north-south component is near zero up to above 700
mbar (^-3.0 km), with the average value being negative
(i.e., from the north) in December to May and just positive
(from the south) in June to November. This is consistent
with surface wind roses presented above.

In the layer between 500 mbar (March to May) or 300
mbar (September to November) and —150 mbar (14.5
km), there are westerly winds on the average in March to
November and weak easterlies in the remainder of the
year. The strongest westerly winds are in the months of

March to May, centered in height around 200 mbar '
(~12.2 km). These are part of the subtropical jet which is
strongest at higher latitudes and earlier in the year. The
north-south component in this layer is, again, quite weak
on the average. Due to the passage of large-scale weather
systems, these mean winds are not representative of the
observed circulation on a given day. Also, the weak
north-south component is particularly sensitive to such
variability, and this is all the more unfortunate as knowl-
edge of this component is fundamental to understanding
the large-scale transport, e.g., of heat and of tracers. While
there are enough upper-air stations in the western tropical
Pacific to make certain our knowledge of this component,
there are vast areas in the mid-latitude Pacific where this
is not so.

There is a quasi-biennial oscillation in the tropical
stratosphere, i.e., the monthly averaged winds shift from
west to east with a period of approximately 26 months in
a band ~-25° latitude wide (full width at half maximum
amplitude) around the equator; this extends from very high
in the stratosphere (>35 km) to at least 100 mbar (—16
km). This is now understood to be an interaction phenome-
non illustrating the coupling between the troposphere and
tropical stratosphere. Its discovery in the early 1960s
illustrates how recently we have begun to learn about this
area of the atmosphere. The "Krakatoa Easterlies," so
named because they were first observed transporting
debris from the spectacular 1883 volcanic eruption, are
not as constant as had been thought.

Sources of Additional Data

Several of the important sources of additional meteoro-
logical data for Enewetak have been referenced in the
previous sections. Here we summarize briefly the
availability of various types of data and indicate the best
sources for discussions on sp>ecialized subjects.

The archive of data used to prepare the figures in this
chapter is the Revised Uniform Summan; of Surface
Weather Observations for Eniwetok Marshall Islands. In
addition to the wind, cloudiness, temperature, and humid-
ity data presented here, it contains extensive information
imfKDrtant primarily for aircraft operations, e.g., ceiling and
visibility data. The document can be obtained for copying
costs from the National Climatic Center, AsheviUe, North
Carolina.

A reliable and useful atlas of tropical wind and tem-
perature charts is included in Newell et al. (1972) along
with sophisticated discussions of the global tropical circula-
tion in dynamical terms. The Pacific island rainfall data and
analysis of Taylor (1973) are an excellent resource. There
is a collection of marine meteorological observations (Sum-
mary of Si/noptic Meteorological Observations, Volume 3,
which includes "Area 10 — Eniwetok") available from the
National Technical Information Service as AD-725 138,
but the data are very sparse.

Solar radiation data are available for certain periods
beginning in May 1977 from the Department of Meteorol-

MERRILL AND DUCE

ogy, University of Hawaii. They are tabulated as hourly
totals of the shortwave radiant energy flux, in cal cm
h~^ Because the data coverage is not continuous (no
period longer than 8 months is available without extended
interruption), it is not presented here.

Many useful and interesting data were collected by
Blumcnstock and Rex (1960) in addition to those discussed
previously.

ATMOSPHERIC CHEMISTRY OF
ENEWETAK ATOLL

Introduction

During the period April to August 1979, an extensive
program investigating the chemistry of atmospheric trace
gases, particles, precipitation, and dry deposition was
undertaken at Enewetak Atoll. The Sea/ Air Exchange Pro-
gram, or SEAREX, was sponsored by the National Science
Foundation and involved efforts by 11 institutions from the
United States, France, and Great Britain. The impetus for
this atmospheric chemistry study was the increasing
interest in the possibility that significant quantities of both
natural and anthropogenic substances may be transported
to the ocean via the atmosphere in mid-ocean regions. An
understanding of the importance of the atmosphere as a
transport path is critical in determining the basic geochemi-
cal cycles and budgets of a variety of naturally occurring
substances and in predicting the near-global impact of
anthropogenic material in open ocean regions. The objec-
tives of the study were to investigate the concentrations
and sources of selected inorganic and organic substances in
the marine atmosphere at Enewetak, their flux into the
ocean, and the mechanisms of their exchange with the
ocean. Substances investigated included trace metals such
as lead, cadmium, zinc, selenium, copf)er, iron, antimony,
manganese, mercury, silver, aluminum, and the alkali and
alkaline earth metals; soil dust; atmospheric sea salt; ^'"^b
and its daughter ^'°Po; particulate organic carbon; and
organic compounds such as PCBs, DDT, aliphatic hydro-
carbons, phthalate plasticizers, fatty acids, fatty and poly-
cyclic alcohols, and low molecular weight ketones and
aldehydes.

The atmospheric chemistry studies at Enewetak Atoll
took place on Bokandretok Island, just north of Enewetak
Island (Fig. 5). During late November and December 1978,
an 18-meter-high walk-up sampling tower and three small
buildings were constructed on Bokandretok. The sampling
tower, located directly on the east coast of the island, was
necessary to get above any local contamination from both
man-made sources and natural sources such as erosion
products and surf spray generated when waves strike the
shoreline.

Additional precautions were taken against local con-
tamination. Sampling pumps were located on the ground
and were connected to the collection systems on top of the
tower by 20 meters of hose. The operation of the pumps

^<f

.BOKANDRETOK

TOWER /

site/

MID-PACIFIC

RESEARCH

LABORATORY

Fig. 5 SEAREX tower site on Bokandretok Island, just
northeast of the Mid-Pacific Research Laboratory on
Enewetak.

was controlled automatically as a function of local wind
speed, direction, and total condensation nuclei in the
ambient air. Pumps were shut down when the wind direc-
tion could cause local contamination from Bokandretok or
other islands in the atoll, when the speed was less than
2.5 ms ', or when the condensation nucleus count was
greater than 300 to 400 cm"^, a typical background level
for marine air. The air sampling tower on Bokandretok is
shown in Fig. 6.

The SEAREX experiments were scheduled to begin in
early January 1979. However, on Jan. 5, 1979, Typhoon
Alice struck Enewetak with winds over 50 ms and very
high tides. The SEAREX tower and one building on
Bokandretok survived but with some damage. The remain-
ing buildings and the submarine cable supplying power to
Bokandretok were destroyed. The experiments were
delayed until repairs could be made, and sampling began in
April 1979.

Atmospheric Sea Salt

The ocean is the largest source for particles, on a mass
basis, in the global atmosphere. These sea salt particles
are produced when wave-produced bubbles burst at the
ocean surface (Woodcock, 1953; Blanchard, 1963). Con-
centrations of this sea salt arc extremely high immediately
downwind of surf breaking on a reef or a shoreline. In
these areas, atmospheric sea salt concentrations can easily

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

».-i^

Fig. 6 SEAREX atmospheric sampling tower in operation on Bolcandretolc Island in 1979.

approach 1 mg m" up to several meters downwind from
the surf line (Duce and Woodcock, 1971). Samples of
atmospheric salt collected from the top of the tower at
Enewetak were not affected by the surf zone, as the high
concentrations produced on the shoreline were carried
beneath the top of the tower by the strong trade winds.
Thus the atmospheric salt concentrations measured on the
tower were representative of concentrations expected over
the open ocean at that elevation (18 m) above the sea sur-
face.

Measured 18-meter-high salt concentration at Enewetak
ranged from about 15 fiq m~'^ at winds of 5 to 6 ms~' to
about 30 fiQ m^"^ at winds of 10 ms~' (McDonald et al.,
1982). The higher winds result in more wave and whitecap
activity and thus higher atmospheric salt concentrations.

The mass median radius of the sea salt collected on
the tower was 3 to 4 /zm at the mean observed relative
humidity of 80%, and generally 90% or more of the salt
was present on particles with radii greater than 1 nm.
McDonald et al. (1982) showed that the rate of deposition

MERRILL AND DUCE

of the salt to the island surface was also very wind speed
dependent but increased with wind speed much more
rapidly than the sea salt concentration itself (Table 3).

TABLE 3

Atmospheric Sea Salt Concentrations
and Deposition at Enewetak'

Wind Atmospheric Atmospiicric

speed, salt concentration, salt deposition.

Mg m

ng cm

3.4t
6.5
10

3
16
29

170
410

'Measured 18 m above sea level (McDonald
et al., 1982).

fS 4 ms ' data obtained from Pigeon Key,
Florida

These authors pointed out that this is because larger salt
particles are present in the atmosphere at higher wind
speeds, and these larger particles have a much higher
deposition, or settling, velocity than the smaller particles.
With their much shorter atmospheric residence times, a
relatively low concentration of larger particles can dom-
inate the flux of the entire particle population. For exam-
ple, at a wind speed of 2.4 ms ', particles with a radius
(at 80% relative humidity) of 4.5 Mm or smaller account
for 60% of the total salt mass but less than 10% of the
salt deposition, while salt particles with radii greater than
10 ^lm account for only 13% of the salt mass but 70% of
the depKjsition.

Asian Soil Dust

The geographical distribution of some mineral com-
ponents, such as quartz and illite, in North Pacific marine
sediments suggests that the atmosphere may be a very
important transport path for mineral matter, or soil dust,
to mid-latitude (30° to 40°N) areas of the North Pacific.
There are, however, few data available on the dust con-
centration in the atmosphere over the North Pacific and no
direct information on the atmospheric input rate of this
material to the ocean surface. During the SEAREX experi-
ments, air filter samples were collected for dust analysis.
The atmospheric concentration of aluminum was used as
an indicator of continental dust in these samples, with the
dust containing about 6.5% Al in the aluminosilicate
matrix. The observed concentrations of dust and salt in the
atmosphere at Enewetak are shown in Fig. 7. While the
atmospheric salt concentration remained relatively con-
stant, the dust concentration dropped by a factor of ~-100
from mid-April to early August 1979 (Duce et al., 1980).

The high concentrations of soil dust observed at
Enewetak were unexpected, especially since Enewetak lies
well within the easterly tradewind regime, and the nearest

( )

(_)

(-1

(/>

lOU

•

• ••

Solt

•

. • • •

■

■
■ ■

Dust

■

■
■

APRIL I MAY I JUNE I JULY I AUG I SEP I

SAMPLE COLLECTION DATE, 1979

Fig. 7 Atmospheric concentrations of dust and sea salt at
Enewetak l)€tween April and August, 1979.

continental land mass, Asia, is about 5000 km to the
northwest. The dramatic decrease in dust over the
5-month period was also unexpected, but both these obser-
vations can be explained on the basis of the seasonal
changes in the large-scale wind patterns over the North
Pacific and the seasonal character of dust storm activity in
the Takia Makan, Gobi, and Ordos Desert regions of
China. Dust storm activity is apparently greatest in the
spring in China due to the combined effects of low rainfall,
the increased occurrence of high surface winds associated
with strong cold fronts, and soil freshly plowed for plant-
ing. The mean surface winds from March through May are
strong easterlies over the western North Pacific between
30°N and the equator; north of 30°N, the surface winds
are weak, with a tendency toward being westerly. How-
ever, at 700 mbar (about 3000 m) there is very strong
westerly flow north of about 20°N extending from well
within Asia to the central North Pacific. Thus dust raised
over China could easily be transported by the mean winds
at this level to the region north of Enewetak. During June
through August, however, conditions are not favorable for
the transport of dust to the central North Pacific. Surface
winds are easterly from Enewetak northward to about
40°N. At 700 mbar the northern boundary of the easter-
lies is located at about 30°N. Persistent westerlies appear
at 700 mbar only north of 40°N, and they are very weak.
Thus we would generally expect much higher atmospheric
soil dust concentrations and deposition rates to the ocean
at Enewetak in the late winter and spring than the rest of
the year. In corroboration of this, Ing (1972) documented
an April 1969 dust storm over China, and satellite photos
showed that dust cloud moving well out over the East
China Sea.

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

The observed mass median radii for the Asian dust in
the SEAREX study at Enewetak ranged from 0.7 to
1.0 /im, considerably smaller than the atmospheric salt
particles. Eighty to 85% of the mass of the dust was
present on particles with radii between 0.2 and 2 ^m. This
is consistent with a very long atmospheric transport path.

Removal of dust to the ocean by rain and dry deposi-
tion was estimated at Enewetak through the analysis of
rain samples and samples obtained by the exposure of flat
plates on top of the tower. The total (wet and dry) deposi-
tion of dust during May 1979 was estimated as about
4 ^g cm - Assuming this deposition was applicable for 3
to 5 months during the spring and early summer, with
somewhat lower deposition the rest of the year, leads to
an estimated annual atmospheric dust deposition to the
ocean near Enewetak of 15 to 30 ^g cm~^ (Duce ct al.,
1980). Settle and Patterson (1982) report dust in rain and
dry deposition at Enewetak which converts to a yearly flux
of about 13 and 1 /:ig cm~^ respectively, the latter being
recycled in sea spray and not contributing to net input.
These inputs can be compared with an estimate of the
annual nonbiological marine sedimentation rate to the
ocean floor in that region of about 50 ng cm^^
(M. Leinen, personal communication). Within the uncertain-
ties in both estimates, it is clear that the atmosphere is a
significant transport path for the nonbiological material
found in marine sediments near Enewetak. It is also clear
that the transport of Asian derived substances to the
Enewetak region is seasonal.

Lead-210 was also measured in the atmosphere at
Enewetak. Lead-210 is a radioactive nuclide produced in
the atmosphere by the decay of gaseous Rn, which in
turn is derived from continental soils. Atmospheric ^'"Pb
was found to decrease over the April to August 1979
period in a manner similar to the atmospheric Al concen-
tration. Lead-210 in air ranged from about 4 dpm per
1000 m^ in April to 0.8 to 1.0 dpm per 1000 m^ in late
July and August (Turekian and Cochran, 1981a, b). Using
^b as an indicator of Asian dust transport, Turekian and
Cochran (1981a, b) calculated a dust deposition of about
10 /ig cm~ yr~ to the ocean at Enewetak.

Trace Metals

A number of trace metals were investigated on parti-
cles in the atmosphere at Enewetak. Some of these trace
metals, e.g., Na, Mg, K, and Ca, were clearly derived from
the ocean as part of the atmospheric sea salt. Interelemen-
tal concentration ratios among this group were the same
as found in sea water. Another group of metals was clearly
associated with the mineral aerosol or Asian dust. This
was determined by using the Al content of the particles as
a reference element for crustal weathering products and
comparing the metal/Al ratio on the aerosols to the aver-
age metal/Al ratio in the earth's crust. An enrichment fac-
tor relative to the crust, EFj_i,,, can be defined as follows:

where (X/AO^,, and (X/AO^^,,, refer to the mass ratio of
metal X to aluminum in the Enewetak aerosols and the
earth's crust, respectively. Values of EF^rust near 1 for any
metal suggests that crustal weathering is likely its source in
the particles (Duce et al., 1975; Rahn, 1976). EF<^,
values for samples collected at Enewetak are given in
Table 4. From this table it is clear that such elements as
Al, Ta, Sc, Mn, Fe, Eu, Ni, Co, V, Hf, Cr, Th, Cu, and Rb
are primarily found associated with mineral or soil aerosol
particles at Enewetak. Metals with an EF,-^^ value higher
than 4, e.g., Zn, Cs, Sb, Ag, Pb, Cd, and Se, apparently
have some source other than continental weathering.

TABLE 4

Geometric Mean EFct,,,, Values for
Atmospheric Trace Metals at Enewetak*

Metal

EF„

0.7 ±

lit

1.8 ± 1.2

0.8 ±

2.0 ± 1.1

0.9 ±

2.3 ± 1.6

1.0

3.0 ± 1.2

1.0 ±

1.1

4.6 ± 1.1

1.0 ±

4.8 ± 1.1

1.0 ±

1.1

27 ± 1.3

10 ±

44 ± 1.8

1.6 ±

1.6

45 ± 2.9

1.6 ±

1.1

Cd
Se

57 ± 4.8
3000 ± 1.9

E'cnist

(X/Al),
(X/A1)„

•From Duceet al., 1981.
fGeometric standard deviation.

On the basis of the measurements made at Enewetak
in 1979, Table 5 presents the expected mean atmospheric
concentrations for a number of trace metals during the
March to June (high Asian dust) period and during the rest
of the year (Duce et al., 1981). Concentration units are
ng (10^^ g) and pg (10"'^ g) per cubic meter of air. Note
that the concentration of all the metals is higher in the
spring than the rest of the year, although the increase in
concentration for many metals during the spring is not as
great as for the metals clearly associated with the dust.
For example, while the mean dust associated metals are
—25 times higher in the spring, the difference for Pb is
less than a factor of 2, Se is about 2, Cd is 5, etc. We
assume the source of these "enriched" metals is also pri-
marily continental regions. Metals associated primarily
with the desert dust have considerably higher
concentrations during the spring and early summer due to
both stronger source functions (i.e., more frequent dust
storms) and wind fields which are conducive to effective
long-range transport to Enewetak during that period.
However, the enriched elements may have continental
sources which are not so seasonal in nature but which are

MERRILL AND DUCE

TABLE 5

Mean Atmospheric Concentrations
of Trace Metals at Enewetak*

TABLE 6

Estimates of Annual Atmospheric Deposition
of Trace Metals to the Ocean at Enewetak"

March-June,

Rest of year,

ng m"'

ng m"^

0.04

Bat

0.02

pgm^

pgm"^

200

0.1

0.5

0.2

200 (5500t)

40 (60t)

0.1

120

250

150(230t)

100(120t)

200

100

< 1

'From Duce et al., 1981, except as

noted.

tF.

'om Settle and Patterson. 1982.

more uniformly distributed throughout the year. Thus their
smaller change in concentration from spring to the rest of
the year may largely reflect the changes in atmospheric cir-
culation patterns for those time periods.

From measurement of these trace metals in rain and
dry deposition, estimates can be made of their atmospheric
deposition to the ocean surface at Enewetak (Duce et al.,
1981). The temfxsral variation in atmospheric concentra-
tions shown in Table 5, the monthly rainfall amounts at
Enewetak (Fig. 2a), and the measured concentrations of
these metals in rain and dry deposition were taken into
consideration when the total deposition rates given in
Table 6 were calculated. Note that the data in Table 6
suggest that both wet and dry deposition arc important for
all elements. There is evidence, however, that much of the
measured dry deposition of at least some of these metals
may be the result of metals being recycled from the sea
surface on sea salt aerosols (Duce, 1982; Settle and
Patterson, 1982). This would mean the dry deposition
values do not represent a net input of these metals to the
ocean. Thus the numbers presented in Table 6 probably
represent an upper limit relative to net inputs from the
atmosphere to the ocean.

Atmospheric deposition

Marine

sedimen-
tation

Wet

Dry

Total

Al, fig cm^^

0.4

3.3

Fe, ng cm~^

1.0

0.3

2.0

V, ng cm~^

Sc, ng cm"^

0.08

0.4

0.9t

Cr. ng cm'^

Eu, ng cm~^

004

0.006t

0.05

0.05t

Cs, ng cm~^

0,3

0.08t

0.4

0.2t

Th, ng cm~^

0.7

0.2t

0.4t

Ta, ng cm"

0 03

0.008

0.04

0.08t

Hf, ng cm~^

0 1

0.03

0.13

o.it

Rb, ng cm'^

1.6

Cu, ng cm~^

Mn, ng cm~^

250

Co, ng cm~^

0.5

o.it

0.6

Pb, ng cm"^

8(6:f)

~4(6t)

~12(12t)

0.61I§

Zn, ng cm"^

-7

-22

Cd, ng cm~

<0.8

-1

0.0211

Se. ng cm~^

0.00711

Sb, ng cm~^

0 12

0.12"

0.24

0.0211

Ag, ng cm"^

0.2

-0.5

-0.7

—

'From Duce et al , 1981.

tEstimated from Al.

tFrom Settle and Patterson, 1982

HEstimated from average marine clay composition.

§Sum of 0.3 authigenic plus 0.3 silicate lattice.

"Estimated from Pb.

Marine sedimentation rates for these metals are
presented in Table 6 and have been determined from the
chemical analysis of surface sediments collected near 29°N
159°W. An estimate of the overall sedimentation rate near
Enewetak was determined from mapping measured sedi-
mentation rates over the entire North Pacific (M. Leinen,
personal communication). Where the surface sediments
were not analyzed for a psarticular metal, crustal ratios to
Al were used for elements present in crustal abundance in
the atmosphere, and average marine clay composition was
assumed for the atmospherically enriched elements.

It is apparent that the atmospheric deposition to the
ocean and the marine deposition to the sediments are very
close for Al, Fe, V, Sc, Cr, Eu, Cs, Th, Ta, Hf, Rb, and
Cu, suggesting atmospheric transport is very important for
marine sedimentation of these metals near Enewetak.
Atmospheric input accounts for only a small part of the
Mn and Co in the sediments. However, the atmospheric
input of Pb, Zn, Cd, Se, and Sb to the ocean is apparently
considerably greater than the deposition of these elements
to the sediments. There are at least two pxjssible explana-
tions for these latter results. First, this would be expected
if the atmospheric concentrations and deposition rates of

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

these metals had resulted from pollution sources on the
continents, since the marine sedimentation rates for these
metals are mean rates applicable to approximately the past
15,000 years. Input of p)ollution-derived trace metals,
which has developed significantly only in the past 50 years
or so, would not be reflected in the measured marine sedi-
mentation rates. Schaule and Patterson (1982) proposed
that a shift may have occurred from principally fluvial
inputs of lead to the oceans in earlier times to primarily
atmospheric input in recent times. Second, these results
would also be expected if a significant fraction of the
atmospheric deposition of these trace metals came from
their recycling from the ocean surface into the atmosphere
and back to the ocean. Recent studies (Weisel, 1981) sug-
gest that recycling of marine-derived metals probably does
not account for more than a few percent of the mass of
these metals in the atmosphere at Enewetak. However,
since these ocean-derived metals would be found on the
large sea salt particles, their dry dep>osition back to the
ocean surface could be rather high (Duce, 1982). Thus,
while it is believed that most of the mass of the enriched
trace metals in the atmosphere at Enewetak is derived
from the continents and very possibly from pollution
sources, a significant fraction of the gross dry deposition of
these metals into the ocean from the atmosphere may be
due to recycled metals from the ocean surface, as
mentioned above.

Lead isotope ratios reported by Settle and Patterson
(1982) confirm that, during the high dust period in April
1979, the pollution-derived Pb had an Asian origin (Tables
7a and b). However, as the Asian dust decreased, the

TABLE 7a

^Pb/^'Pb Ratios in
Filtered Air Sample at Enewetak'

Collection
date, 1979

*Pb/^Pb

4/22 to 5/09
5/09 to 5/15
7/12 to 8/10

1.170
1.196
1 205

^°^Pb/^''^Pb ratio increased and became similar to that for
pollution-derived Pb from North America. Thus some, if
not most, of the small particle pollution-derived Pb found
at Enewetak in the summer may have been transported
from North America to Enewetak. On the basis of "Tb
and stable lead measurements, Settle et al. (1982) and Set-
tle and Patterson (1982) calculated a net atmospheric
stable lead deposition rate of 4 to 10 ng cm~ yr at
Enewetak. This agrees well with the value of 8 to 12 ng
cm~^ yr~^ given in Table 6.

The mass median radii (MMR) for the particles contain-
ing the various trace metals are presented in Table 8.
Note that the sea salt metals (Na, Mg, K, and Ca) have
MMRs near 3.5 /zm while the crustally derived metals have
MMRs of 0.75 to 1.0 nm. The enriched metals (Zn, Se,
Sb, and Pb) have MMRs of <0.5 ^m, consistent with a
possible pollution source for these elements.

TABLE 8

Mean Particle Mass Median Radii (MMR)
for Trace Metals at Enewetak*

MMR.

Element

fim

Element

3.4

0.80

3.5

1.1

3.4

0.84

3.5

0.74

0.80

0.75

0.72

0.88

0.81

0.76

0.25

0.96

0.53

0.70

-0.35

0.79

0.04

'From Duce et al., 1981.

Gaseous and particulate mercury were also investigated
at Enewetak (Fitzgerald et al., 1981). The concentrations
observed are given in Table 9. It is apparent that mercury
exists almost entirely as a gas at Enewetak. The relatively
small temporal variation in gaseous Hg concentration (and
the fact that similar concentrations are found at other
marine areas) suggests a relatively long atmospheric

TABLE 7b

^Pb/^'Pb Expected from
Major Continental Sources*

Region

*Pb/^Pb ratio

Asia/ Japan 1.153 to 1.165

USA. 1.190 (1974) to 1.230 (1978)

Mexico 1.187

'From Settle and Patterson, 1982.

TABLE 9
Atmospheric Mercury at Enewetak'

Collection

period,

1979

Gaseous Hg, Particulate Hg,

ng m

ns m

4/27 to 5/21
6/28 to 8/6

1.6 ±0.6
1.7±0.5

0.0005
00012

•From Fitzgerald et al., 1981.

MERRILL AND DUCE

residence time for the vapor phase. Studies of Hg specia-
tion in the atmosphere at Enewctak indicate that the gas
phase is principally inorganic mercury, of which elemental
Hg is probably the major component. Mercury in rain at
Enewetak was found to have a concentration of 2 ng 1
Apparently this concentration is derived primarily from the
washout of the particulate Hg rather than the vapwr phase.

are reported in Table 11. It is apparent that the vapor
phase dominates the concentration of these compounds, at
least from n-C2i to n-C^Q, and probably for the lower car-
bon number alkanes as well. Duce and Gagosian (1982)
used the concentration distribution in Table 11 to model
the input of particulate nalkanes (n-Ci5 to n-C3o) and
vapor phase nalkanes (n-Cjo to n-Cao) from the atmo-

Organic Carbon

The organic carbon concentration of atmospheric f)arti-
cles at Enewetak was ~0.9 ^g m~^ (Chesselet et al.,
1981) (Table 10). This is typical of marine regions, where
the concentration generally ranges between 0.2 and 1.2 /xg
m""^. Eighty to 85% of the mass of this organic carbon at
Enewetak is found on particles with radii less than 1 nm.

TABLE 10
Atmospheric Organic Carbon at Enewetak

Concentration

Mg m

Reference

Particulate 0 89 + 017

Rain

Chesselet et al.,
1981
0.64 ±0 48 Gagosian et al.,
1981b

Carbon isotope studies by Chesselet et al. (1981) have
suggested strongly that the small particle (<1 ^m) organic
carbon does not have a marine origin. By measuring both
C and C, one can calculate 5 C as follows:

5'3c =

\ ^/ *^/sample

/13/-;12,

C/ CJstandard

-1

X 1000

5 C values calculated for the smallest particles (r < 1 /xm)
are -26°/oo to -28°/oo. Chesselet et al. (1981) point
out that this range is similar to b^^C values for continental
vegetation, coal, and the products of jsetroleum combus-
tion, — 26 ± 2°/oo, suggesting the small particle carbon is
of continental origin. The d^'^C values for the larger parti-
cles (r > l^im) are -187oo to -22° I ^. This is similar to
the 6 C value for marine organic carbon, which is gen-
erally -21 ± 2°/oo in low latitude regions (40°S to
50°N), suggesting the large particle carbon in the
Enewetak marine atmosphere is of marine origin.

The organic carbon content of rain at Enewetak aver-
aged 1.2 mg l~^ during April and May and 0.3 mg 1
during July and August 1979 (Gagosian et al., 1981b).

Organic Lipid Class Compounds

Particulate and vapor phase heavy n-alkanes were
measured independently in the atmosphere at Enewetak by
two research groups in 1979. The observed concentrations

TABLE 11
Atmospheric N-Alkancs at Enewetak

Concentration

Particulate.*
ng m^

Gaseous

N-alkane

ng m~^"

ng m *t

nCi3

0.23

n-Ci4

0.19

n-Ci5

0.66

nCi6

0.13

n-Ci7

0.55

n-Ci8

0.07

n-C,9

0.07

n-C2o

007

n-Cji

0.0017

0.07

n-C22

0.0020

0,07

n-C23

00023

0.09

n-C24

00021

0.12

n-C25

0.0030

0 095

0.12

n-C26

0.0020

0.088

0.09

n-C27

00067

0055

007

n-C28

0.0037

0.024

n-C29

0.0170

0.019

n-Cjo

0.0033

0.013

'Gagosian et al , 1981b, 1982.
fAtJas and Giam, personal communication,
1981.

sphere to the ocean. Consideration was given to rain
scavenging of both aerosol and vapor phase n-alkanes, dry
depKDsition of aerosol n-alkanes, and direct gas exchange
with the ocean of vap>or phase n-alkanes. Estimates of the
atmospheric input of n-alkanes to the ocean at Enewetak
are given in Table 12. Note that rain scavenging of n-
alkanes on particles appears to be the primary method of
n-alkane removal from the atmosphere.

It can be seen from Table 11 that the odd carbon
number n-alkanes on aerosols have higher concentrations
than the adjacent even carbon number n-alkanes. This is
observed for higher n-alkanes up to n-Cae as well (Gago-
sian et al., 1980). The odd-to-even carbon preference
index and the fact that the major alkanes are n-C27. n-C29,
and n-Csi strongly suggest that the source of these heavier
n-alkanes present on aerosols is vascular plant waxes,
probably of Asian origin (Gagosian et al., 1981a).

Concentrations of fatty alcohols, fatty acid esters, and
fatty acid salts were also measured in the Enewetak atmo-
sphere and are presented in Table 13 (Gagcsian et al..

METEOROLOGY AND ATMOSPHERIC CHEMISTRY

TABLE 12

Estimate of Annual Atmospheric Deposition of
n-Cio to n-Cao Alkanes to the Ocean at Enewetak"

Deposition
mechanism

Deposition rate,
lO'^gcm^yr'

Particulate:
Wet
Dry

VafKjr Phase:
Wet
Dry

Total

6.2 to 62
0.8 to 8

0 to 0.00001
Oto 1.4

7 to 71

'From Duce and Gagosian, 1982.

TABLE 13

Concentration of Fatty Alcohols, Fatty Acid Esters, and
Fatty Acid Salts on Atmospheric Particles at Enewetak"

Organic
substance

Concentration range,
pg m~^

Fatty alcohols

C21"C32

Fatty acid esters
C13-C20

C21~C32

Fatty acid salts
C13-C20

C21~C32

2 to 85

58 to 210

34 to 290

6 to 91

87 to 4000

36 to 670

'From Gagosian et al., 1981b.

1981b, 1982). These authors suggest, on the basis of
odd/even carbon concentration ratios and the concentra-
tion distribution observed, that the fatty alcohols and the
C21 to C32 fractions of the fatty acid esters and fatty acid
salts have a natural terrestrial source, whereas the lighter
Ci3 to C20 fatty acid esters and fatty acid salts in the
Enewetak atmosphere likely have a marine source.

Heavy Chlorinated Hydrocarbons and
Other Synthetic Organics

A number of synthetic organic substances were mea-
sured in the Enewetak atmosphere, including PCBs and
certain pesticides and plasticizers (Atlas and Giam, 1981).
The concentrations observed in the vapKjr phase are
presented in Table 14. Concentrations of these substances
on aerosols were less than 10% of the vapor phase con-
centrations. Altas and Giam (1981) point out that rain may
not be the primary mechanism for removal of these vapor
phase organic substances from the air. Direct vapor
exchange with the ocean may be most important. By using
a vapor deposition velocity of 8 m h ' for PCB 1242,
the direct vapor exchange PCB flux to the ocean at
Enewetak would be 35 X 10 '° g cm"^ yr"^ This is at
least 50 times greater than the precipitation flux of
<0.8 X 10"'° g cm^^ yr"' for Aroclor 1242, which can
be calculated using the concentrations in Table 14 and the
annual rainfall amount in Fig. 2a. The vapor phase flux
would result in a PCB Aroclor 1242 atmospheric residence
time of about 30 days, which would explain the relatively
uniform concentration distribution of Aroclor 1242 over
the Atlantic and Pacific Oceans and nonurban continental
areas (Atlas and Giam, 1981).

Phthalic acid esters are present in rather high concen-
trations at Enewetak. These compounds are widely used as
plasticizers. The concentrations are similar in air over the
Atlantic Ocean, the Gulf of Mexico, and in a rural area in

TABLE 14

Concentration of Synthetic Organic Compounds
in the Air and in Rain at Enewetak'

Air concentration.

Rain concentration.

ng m

ngl '

Compound

Mean

Range

Mean

Range

PCB, Aroclor 1242

0.54

0.35 to 1.02

<0.6

PCB, Aroclor 1254

0.06

Hexachlorobenzene

0 10

0.095 to 0.13

<0.03

a Hexachlorocyclohexane

0.25

0.075 to 0.57

3.1

1.3 to 6.8

7 Hexachlorocyclohexane

0.015

0.006 to 0.021

0.51

0.34 to 1.6

Chlordane (a and 7)

0.013

0 006 to 0.015

<0.02

Dieldrin

0.010

0 006 to 0.018

<0.02

p.p'-DDE

0.003

0.002 to 0.005

<0.02

Di-n-buthyl phthalate

0.87

0.40 to 1.8

2.6 to 73

Di-(2-ethylhexyl) phthalate

1.4

0.32 to 2.7

5.3 to 213

'From Atlas and Giam, 1981.

MERRILL AND DUCE

Texas, suggesting relatively long atmospheric residence
times. Urban area concentrations are about 100 times
higher (Atlas and Giam, 1981).

ACKNOWLEDGMENTS

Wc thank the staff of the University of Hawaii's Mid-
Pacific Research Laboratory, the Department of Energy,
and Holmes and Narver, Inc. for field support in Enewetak.
Paul Dellegatto assisted in the reduction of the wind rose
and cloudiness data. Supported by NSF Grants OCE
77-13072, CX:E 77-13071, and OCE 81-11895.

REFERENCES

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Pollutants: Ambient Concentration in the Remote Marine
Atmosphere, Science, 211: 163 165

Blanchard, D. C., 1963, Electrification of the Atmosphere by Par-
ticles from Bubbles in the Sea, Progress in Oceanogr . 1:
71-202

Blumenstock, D 1, 1961, A Report on Typhoon Effects Upon
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, and D F. Rex, 1960, Microclimatic Observations at

Eniwetol^, Atoll Res Bull , 71: 1158,

Chesselet, R , M. Fontugne, P Buat-Menard, U. Ezat, and C. E.
Lambert, 1981, The Origin of Particulate Organic Carbon in
the Marine Atmosphere as Indicated by Its Stable Carbon Iso-
topic Composition, Geophi^s Res Lett., 8: 345-348.

Chapman, S., and R. S. Lindzen, 1970, Atmospheric Tides Ther-
mal and Grauitational. Gordon and Breach Science Publishers,
New York.

Duce, R. A., 1982, Sea Salt and Trace Element Transport Across
the Sea/Air Interface. Presented at the Joint Oceanographic
Assembly, Halifax, Canada, Aug. 4

and R. B Gagosian, 1982, The Input of Atmospheric n-Cio

to n-C3() Alkanes to the Ocean, J Geophys Res, 87:
71927200.

, G. L Hoffman, and W. H Zoller, 1975, Atmospheric Trace

Metals at Remote Northern and Southern Hemisphere Sites:
Pollution or Natural?, Science. 187: 59-61

C. K. Unni, B. J. Ray, P J. Harder, A P Pszenny, and

J. L. Fasching, 1981, The Atmospheric Concentration of
Trace Elements and Their Deposition to the Ocean at
Enewetak Atoll, Marshall Islands, in Svmposium on the Role
of the Oceans in Atmospheric Chemistry, lAMAP Third Scien-
tific Assembly, Hamburg, F.R.G.

, C. K. Unni, B J Ray, J M. Prospero, and J. T Merrill,

1980, Long-Range Atmospheric Transport of Soil Dust from
Asia to the Tropical North Pacific: Temporal Variability,
Science. 209: 1522-1524
, and A. H. Woodcock, 1971, Difference in Chemical Compo-
sition of Atmospheric Sea Salt Particles Produced in the Surf
Zone and on the Open Sea in Hawaii, Tellus, 23: 427-435.
Fitzgerald, W F., G. A. Gill, and A. D. Hewitt, 1981, Mercury,
A Trace Atmosptieric Gas, in Symposium on the Role of the
Oceans in Atmospheric Chemistr\^. lAMAP Third Scientific
Assembly, Hamburg, F R.G.

Gagosian, R B , E. T Peltzer, and O. C. Zafiriou, 1980, Und-
Derived Organic Compounds in Enewetak Particulate Sam-
ples, SEAREX Newsletter. 3: 10-15.

, E T. Peltzer, and O. C. Zafiriou, 1981a, Atmospheric

Transport of Continentally Derived Lipids to the Tropical
North Pacific, Nature, 291: 312-314.

E. T. Peltzer, and O C Zafiriou, 1981b, Organic Com-
pounds in Vapor Phase and Rain Samples from the Enewetak
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O. C Zafiriou, E T. Peltzer, and J. B. Alford, 1982, Lipids

in Aerosols from the Tropical North Pacific: Temporal Vari-
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Lavoie, R L., 1963, Some Aspects of the Meteorology of the
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1-80.

McDonald, R. L., C K Unni, and R. A. Duce, 1982, Estimation
of Atmospheric Sea Salt Dry Deposition: Wind Speed and
Particle Size Dependence, J Geophys. Res . 87: 1246-1250.

Newell, R. E., J. W. Kidson, D. W. Vincent, and G. J. Boer,
1972, The General Circulation of the Tropical Atmosphere
and Interactions with Extratropical Latitudes, Vol. 1, MIT
Press, Cambridge, Massachusetts.

Rahn, K. A., 1976, The Chemical Composition of the Atmo-
spheric Aerosol, Technical Report, Graduate School of
Oceanography, University of Rhode Island, Kingston, Rhode
Island

Reed, R J , and E E Recker, 1971, Structure and Properties of
Synoptic-Scale Wave Disturbances in the Equatorial Western
Pacific, J Atmos Sc> . 28: 11171 133.

Schaule, B K., and C. C. Patterson, 1981, Lead Concentrations
in the Northeast Pacific: Evidence for Global Anthropogenic
Perturbations, Earth Planet. Sci. Lett.. 54: 97-116.

Settle, D. M., C. C. Patterson, K. K Turekian, and J. K.
Cochran, 1982, Lead Precipitation Fluxes at Tropical Oceanic
Sites Determined from ^"^b Measurements, J Geophys.
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, and C C Patterson, 1982, Magnitudes and Sources of Pre-
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8857-8869.

Taylor, R. C, 1973, An Atlas of Pacific Islands Rainfall. Hawaii
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Trenberth, K E., 1976, Spatial and Temporal Variations of the
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Turekian, K. K., and J. K. Cochran, 1981a, The Temporal Varia-
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and J K Cochran, 1981b, ^"^b in Surface Air at Enewetak

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Chapter 7

Subtidal Environments and Ecologx; of
Eneioetak Atoll

PATRICK L. COLIN

Motupore Island Research Station. Universittj of Papua
New Guinea. Port Moresbi>. Papua New Guinea

INTRODUCTION

The area of subtidal marine environments of Enewetak
far exceeds the intertidal and terrestrial habitats. Subtidal
environments are the lagoon and outer reefs and the pas-
sages between them which arc submerged at low tides.
The lagoon and outer reefs are separated, except at
passes, by the intertidal reef flat. Although closely con-
nected to the subtidal habitat, the intertidal habitat of
Enewetak is not discussed in this chapter except as it
relates to circulation, energetics, and processes in the sub-
tidal environment. With an area of about 930 km , the
lagoon is more than 15 times the area of the intertidal reef
flat and 140 times the area of the islands.

The Enewetak Lagoon is relatively deep by atoll stan-
dards (Wiens, 1962), with a mean depth of 48 m and a
maximum depth of 64 m. Only about 10% of the lagoon is
shallower than 18 m, and only 20% is less than 32 m
(Emery et al., 1954). The open waters of the lagoon are a
voluminous habitat of about 4.2 X 10^° m^ of water,
most of which overlies the deep portions of the lagoon. In
both area and volume, the lagoon is the largest subtidal
unit at Enewetak. For this chapter, an arbitrary depth of
30 m has been selected to distinguish between the "deep"
and "shallow" portions of the lagoon.

The area of the reefs seaward of the reef flat and
islands has never been accurately determined. Based on
reef widths observed from the air compared to adjacent
reef flat widths, the area of the seaward reefs is certainly
less than that of the reef flat, perhaps by a factor of up to
3 or 4, but an accurate determination is not presently pos-
sible.

The outer slope of the atoll is quite different from the
lagoon. The present discussion will include descriptive
information on the outer slope to 300 to 400 m depth,
but below those depths there is little detailed information
concerning the biological communities or geological per-
spectives.

The subtidal environment of Enewetak contains a
number of units, divisible on the basis of location, physical
factors, substrate types, dominant organisms, and other
factors. Biological communities can be similarly identified.
The generally high diversity of marine organisms at
Enewetak increases the complexity in describing individual
communities and their relationship with the others.

MARINE CONDITIONS

Mean surface oceanic water temperatures at Enewetak
range between 27° and 29°C (Atkinson et al., 1981;
Coles et al., 1976), with March the coolest month and
August the warmest. Temperature extremes during any
one month vary about ± 1°C from the mean (Coles et al.,
1976). Local conditions can alter these, with isolated tide
pools in the midday sun reaching the low 40s C.

Typical temperature-depth profiles for the seaward
reefs of Enewetak during summer are shown in Fig. 1.
From the surface to about 125 m in depth, the tempera-
ture gradually decreases from 29° to about 25°C. A slight
thermocline begins at about 125 to 150 m, and it changes
from 25° to 20°C over a 30 to 40 m increase in depth.
At 220 m it is about 13°C with the temperature gradually
decreasing to about 9°C at 380 m.

Atkinson et al. (1981) documented the isothermal and
isohaline nature of the lagoon water column with no more
than a 0.5°C variation in temperature and a 0.20 ppt
range in salinity. Almost without exception the shallow
waters of the open lagoon and ocean are ideal for eury-
thermal tropical organisms. The salinity of the lagoon is
essentially the same as that of the open ocean. Only in
areas of restricted circulation and shallow depth are tem-
peratures elevated significantly above genera! lagoon
water.

The atoll is located in the North Equatorial Current
with a general westward movement of water past the atoll.
Currents observed on the windward (east) ocean reefs
agree with this generalized picture, even at depths of 100
to 200 m, with the ocean current seeming to split north
and south near the easternmost extension of the atoll at
Ananij. On the ocean side of Enewetak Island, the
alongshore component of the current varies in speed but

COLIN

TEMPERATURE C.
25° 30° 30°

Fig. 1 Tempcrature/dcpth profiles from four dives by the
DSRV Makali'i at Enewetalt Atoll, summer 1981. Locations of
profiles were as follows: a, Biken, west side of the atoll;
b. east side of wide channel; c. Bokandretok, just north of
Enewetak Island; and d. Runlt Island. Dual tracks represent
an ascent and descent profile.

has never been observed to move northward. It is only
near passes into the lagoon that tidal currents potentially
cause reversal of current direction on the ocean slope of
the atoll. On the leeward (west) side of the atoll, currents
are variable, and an eddy pattern often seems to exist. At
the West Spit, the extreme northwest tip of the atoll, the
currents from north and south seem to converge.

Currents in the lagoon and passes are considerably dif-
ferent and are covered in detail by Atkinson et al. (1981)
and in Chapter 5 of this volume. In the lagoon the surface
current is generally a wind-driven westerly flow with mid-
depth return flow to the east. Most water enters the
lagoon over the windward reefs and passes out the wide
(south) channel. The deep channel has strong tidal flow but
little or no net input into the lagoon. Water residence
times have a mean of about 30 days but can vary between
a few to more than 130 days. The residence time of water
in the northern p)ortion of the lagoon is greater than the
mean.

The nutrient-poor oceanic water eastward of Enewetak
is clear, with visibility normally exceeding 50 m. Such
water visibility is typical of windward ocean reefs, but visi-
bility changes when water enters the lagoon over the reef
flat. Increased production and suspended particulates
reduce the visibility of lagoon waters to 10 to 25 m and
occasionally less. Aerial photographs of Enewetak have
features on the ocean side reefs visible to depths of about

40 m, although this is limited to a maximum of about 15
to 18 m in lagoon waters. In the northern lagoon, particu-
larly near the islands between Engebi and Bokoluo, two
factors may contribute to visibilities at less than 10 m.
First, lagoon water residence times in this area are near
the maximum, hence high densities of phytoplankton and
zooplankton can develop in this water. Second, the pres-
ence of fine, easily suspended particulates produced as a
result of nuclear tests and cratering in this area may
greatly reduce water visibility. Phytoplankton blooms, man-
ifested both as "brown water" and large, thick windrows of
extremely dense phytoplankton, have been observed on
several occasions in the northwestern lagoon. Visibility in
such waters is consequently extremely limited.

Trade wind conditions with steady 10 to 20 knot
winds from the east to northeast prevail throughout most
of the year at Enewetak. During the summer, trade winds
are usually lighter than during the winter, and they can
cease for periods of several days. The normal trade winds
produce oceanic waves about 1 to 2 m high which hit the
windward reef of the atoll. Within the lagoon, the margin
on the windward side is relatively calm, being protected by
islands and interisland reefs. At high tide, however, much
more wave action comes across the reef flat from ocean to
lagoon, making conditions choppier on the lagoon margin.
The waves which do cross the reef are of small height and
wave length, making the surface rough for small boats but
having little effect below a few meters depth.

Moving westward across the lagoon, a significant fetch
is achieved quickly, and when trade winds prevail, the cen-
tral and western areas of the lagoon are far from placid.
Waves of 1.5 to 2 m with whitecaps can occur, and the
lagoonward edge of the leeward reef can have significant
surf breaking on it. Significant wave action can also reach
the lagoon sides of the islands west of Engebi and the
southwestern islands.

The ocean side of the leeward reefs and islands is calm
under trade wind conditions with the tide level affecting
the wave action crossing the reef from lagoon to ocean. In
the lee of islands it is extremely calm for an oceanic area.

Waves in the Marshall Islands as a whole are from the
east or northeast, a consequence of persistent trade winds.
Waves exceeding 3.5 m high comprise fewer than 2% of
waves in the Marshall Islands area (Japan Meteorological
Agency [JMA], 1971 to 1978). Waves greater than 3.5 m
can occur any time of the year and are generally associ-
ated with (1) local storms or typhoons from the east
through southwest and (2) more distant northern and
southern hemisphere storms. The greatest wave amplitude
observed (JMA report) was a long-period 6.5-m swell from
the northeast.

Local conditions can also greatly affect wave action.
Where tidal currents lun against the trade winds, steep
standing waves develop. The east channel at Enewetak can
be treacherous under strong trade winds with the tide
dropping sharply. A distinct central tongue of breaking
waves extending out the channel to the ocean is visible
from the air under these conditions. Similarly, the west

SUBTIDAL ENVIRONMENTS AND ECOLOGY

side of the wide passage is an area of merging waves from
the lagoon and refracted oceanic waves and swell with
strong southerly currents producing standing waves and
short, steep seas.

During summer calms and at odd times during the rest
of the year, the lagoon and surrounding ocean become
smooth. At such times it is possible to swim off the reef
flat to the windward ocean reefs because the surf is small
and gentle Surface slicks are found in lagoon waters. The
windrows of phytoplankton from blooms have been found
during such calm periods.

Swells from distant storms create a different situation
in which shores exposed to the swell (which can be either
windward or leeward) are heavily pummeled, whereas the
waves produced by wind in the immediate area may be
small. Most impressive are those rare times when large
swells thunder against the reef while calm trade winds pro-
duce an almost mirror like surface elsewhere. On shores
normally lacking high surf, these waves can cause consider-
able damage. Such swells can also enter the lagoon
through the southern pass to break on the lagoon shore of
Enewetak, Medren, and other islands.

Several typhoons and near-typhoon strength storms
have passed by or over Enewetak during the last decade.
Although the atoll is generally not considered in the
typhoon belt, these storms have had a significant and
readily visible effect on Enewetak reefs. Often a storm
whose center does not pass especially close to Enewetak
can produce storm waves which severely damage reefs,
although above-surface damage from winds may be very
light.

Deep Lagoon Biological Communities

The lagoon bottom below 30 m depth consists largely
of soft substratum with small to large reef structures (pin-
nacle reefs) spread randomly throughout the area. Except
for the pinnacle reefs, relatively little published information
exists regarding in-situ observations of either the lagoon
slope or deep lagoon benthic biological communities or
geology. Nearly all published information is based on
surface-lowered grab samplers or dredges. Emery et al.
(1954) reported on results of samples taken by an "under-
way" bottom sampler and about 50 photographs taken by
a remote camera at unspecified locations in the deeper
lagoon. More recent researchers (Nelson and Noshkin,
1973; Noshkin, 1980) have relied on grab samplers or
short-core samplers to obtain bottom samples for analysis.

The only published in-situ observations of the deep
lagoon are those of Gilmartin (1960), made during deep
scuba dives on a transect across the southern lagoon.
Twelve of these stations were below 30 m in depth. All
stations below 30 m had coral patches present within the
range of visibility; these varied from only a few small
heads in one instance to massive patch reefs rising 15 m
or more above the surrounding bottom. Some stations had
the substratum covered with "mounds of sand and cast-
ings," but for most of the deeper stations, the presence or

absence of mounds was not noted. This study confirmed
that abundant algal communities exist in the deep areas of
Enewetak Lagoon, many occurring at the deepest depths
reached (62 m). Coral patches at these depths seemed
more densely populated with algae than adjacent sand.
Eight species of Halimeda were found primarily between
42 and 62 m, supporting previous reports that the genus
was "most common and luxuriantly developed at the
deeper levels." Gilmartin (1960) was the first person to
realize the intense bioturbation of the soft substrate bot-
toms of the lagoon, commenting that "the continual 'churn-
ing' of the substratum by these benthic organisms has
prevented algae, which might occur elsewhere on the same
stations, from starting and growing to the fX)int where they
would not be 'uprooted' or buried by the sand displace-
ments."

During 1980 and 1981, a distributional survey of deep
lagoon benthic communities was conducted using a
lowered camera system. During this "Enewetak Benthic
Survey" over 2000 photographs covering 24 m each
were taken at 190 stations throughout the deep lagoon
(Colin, 1986). Additionally, in the summer of 1981 the
submersible Makali'i was utilized for a series of dives in
several areas of the deep lagoon to augment the photo-
graphic survey.

STATE OF KNOWLEDGE OF SUBTIDAL
MARINE ENVIRONMENTS

With certain exceptions, the subtidal marine environ-
ments of Enewetak cannot be characterized as well known.
Often our knowledge is based on studies in the southern
lagoon close to the lee of the southern islands. The dis-
tances from support bases, the generally rough conditions
of the lagoon outside protected lee areas, and the rapidly
increasing water depth have severely limited work in both
the northern and central lagoon. Much of the work accom-
plished in the southern lagoon is of equal applicability to
the entire lagoon, but differences do exist between these
areas.

Even the southern areas of the lagoon below 20 to 30
m depth are poorly known. This is due to the limited
access of scuba-diving scientists to the deep lagoon bot-
tom, particularly at its most common depths of 40 to 60
m. Although a few hardy souls have ventured to dive in
these areas, working time is limited, nearly precluding
studies providing an understanding of overall conditions in
the deep lagoon. Significant work on the deep lagoon floor
requires either specialized instrumentation and recording
methods or suitable vehicles for in-situ work.

The present work is intended to provide descriptive
information about subtidal habitats in the following order:
(1) deep lagoon; (2) shallow lagoon; (3) lagoon-ocean
passes; and (4) the seaward reefs, from the center of the
lagoon outward. The information has been drawn from
publications, annual reports, unpublished information in

COLIN

MPRL files, and unpublished data from numerous scien-
tists.

No comprehensive descriptive account of the subtidal
environments of Enewetak has been attempted. Only a few
specialists have endeavored to discuss atollwide distribu-
tion and contributions of their restricted groups of organ-
isms. Cuffey (1973 to 1978) examined the role of bryozoa
at Enewetak with comparison to other reef areas. He listed
three major marine benthic "macrohabitats" at Enewetak:
the coral-dominated, the bedrock-dominated, and the
sediment-dominated. He distinguished between "larger
patch reefs (from 25 to more than 150 ft height)" and
"smaller coral knolls (from 1 to 25 ft high)" in examining
bryozoan distribution at Enewetak. He also distinguished
between biohcrms ("coral-dominated macrohabitats with
significant height") and biostromes ("coral-dominated
macrohabitats lacking significant height"), such as his
"coral pavement."

Allen (1972), in his work on anemonefishes, provides
brief descriptions of some Enewetak habitats. The major
physiographic features of environments from outer reef
slope, reef flat, shallow lagoon margin, and deep lagoon
arc mentioned. He thought that the deep lagoon floor
"appears to be of a fairly uniform nature" and had "large
stretches of sand with orcasional small patch reefs." Deep
lagoon pinnacles were described as "an oasis, rising from
the barren lagoon floor" and harboring "an extraordinary
wealth of marine organisms."

The deep lagoon can be characterized as dominated by
sediment substrates but with reefs of widely varying size
and vertical relief, distributed fairly evenly throughout the
lagoon. The soft substrate supports several different biolog-
ical communities, often occurring within short distances of
other soft substrata and reef substrata. Extensive distur-
bance of the sediments is evident in many of the benthic
photographs and from submersible dives.

Based on point counts of the benthic survey photo-
graphs, approximately 859b of the deep lagoon has soft
substrata, with the remaining 15% hard substratum. Nearly
half of the stations had 100% coverage of soft substrata,
more than 60% were 90% or more soft substrata, and
more than 75% were 75% or more soft substrata cover-
age. If the individual photographs are considered, rather
than entire stations, slightly higher percentages of 100%
and 90% soft substrate coverage are found.

The soft substrate biological communities comprise
four identifiable types. These include (1) open sand sub-
strate without a visible algal mat, (2) sand substrate with
visible algal mat on its surface ("algal film"), (3) sand sub-
strate with macioalgae, particularly species of Halimeda,
on its surface ("algal flat"), and (4) sand with large popula-
tions of an unattached Fungiid coral, Cxjclosersis and
Diaseris spp. ("button corals"). Typical views of these com-
munities from benthic survey photographs are shown in
Figs. 2 and 3. Interpretation of the benthic survey photo-
graphs has been facilitated by observations and photo-
graphs from the Enewetak submersible project and scuba
dives in shallower areas where similar communities occur.

The soft substrate communities often intergrade, for
example, the macroalgae of the "algal flat" community
decreasing in density until only open sand remains. Arbi-
trary points at which one community "becomes" another
have been used in interpreting the photographs, but abso-
lute distinctions among types of soft substrate communities
are often impossible. The distributions of community types
in the deep lagoon as based on benthic survey
photographs are shown in Figs. 4 and 5.

Deep lagoon sediment substrata with no visible algal
cover are qualitatively similar to areas of the lagoon mar-
gin as shallow as 15 m. They are usually heavily
bioturbated, dominated by the conical mounds produced
by callianassid shrimp. The occurrence of open sand sul>-
strates, based on benthic survey photographs, is shown in
Fig. 4. Although this covers only a limited number of sta-
tions, it does indicate "barren" soft substratum can occur
throughout the lagoon. Open sand substratum, however,
can change within a few meters horizontally to soft sub-
stratum covered with macroalgae. Such short-scale horizon-
tal changes among soft-substratum communities and hard
substrata are common throughout the lagoon.

It is possible that the rapid sediment turnover in open
sand areas is responsible for the lack of dark algal films of
macroalgae. However, algal mats over 1 m in diameter do
occur in heavily bioturbated areas but are capable of form-
ing in only a few days time. Biological sediment overturn is
concentrated at specific points in the short term (days) pro-
ducing "splotching" of algal mats when viewed from above.
Other factors affecting density of algal films (nutrients in
water or sediment, water clarity, standing crop already
present, etc.), may be critical in controlling the presence of
dense algal mats.

The presence of algal films, particularly diatoms and
blue-green algae, on sand bottom without visible algal mat
is well known (J. T. Harrison, personal communication).
The population level at which an algal mat becomes visible
in a photograph or to a human observer is dependent on
the standing crop per unit area and the plants involved.

Water visibility over open sand bottoms is often limited
to only 5 to 10 m, even in the deep lagoon. Considerable
amounts of suspended particulates were observed from the
submersible Makali'i, using its lights, in the deep lagoon;
but relative densities varied considerably from day to day
at one location. It was noted, however, that suspended
particulates were often elevated closer to the bottom than
near the lagoon surface. Similar observations commonly
have been made by scuba divers over open sand substrates
at depths around 20 m.

Sediment will often have a clearly visible thin layer of
microalgae on its surface. Algal films are seen in the
lagoon margin areas as shallow as 15 m. At depths of 15
to 30 m, small dense algal mats (only a few centimeters in
diameter) are often seen on otherwise clear bottoms.
Where a film of algae exists, any recent disturbance of the
sediment is clearly indicated by lack of, or disturbance of,
the algal mat. This relationship has been closely examined
at diving depths from 15 to 30 m and has been verified in

SUBTIDAL ENVIRONMENTS AND ECOLOGY

f.-

Fig. 2 Enewetak deep lasoon soft bottom communities. Bar equals approximately 1 m. a, Algal film community photographed
by vertical benthic camera. Considerable bioturbation (pale splotches) is visible in this photograph (56 m depth); b, Algal flat com-
munity, vertical l)€nthic camera, dominated by species of Halimeda (51 m depth); c. Algal flat community dominated by mixed
macroalgae, vertical benthic camera (55 m depth); d. Algal flat community at 57 m depth, central lagoon, dominated by mixed
macroalgae, with small Halimeda sp. thalli. Some mounds produced by bioturbation are visible (diver photograph); e, Same gen-
eral area as above with sponges (dark objects near center), Halimeda sand and abundant macroalgae visible, 57 m depth (diver
photograph): f. Near vertical view of area of Fig. 2d and 2e, 57 m depth. Significant bioturbation is evident in this diver photo-
graph. [Figures from Colin, 1986.]

COLIN

Fig. 3 Enewetak deep lagoon communities. Bar equals approximately 1 m length, a. Algal flat community dominated by
Caulerpa, vertical benthic camera photograph (51 m depth): b, Oblique view of sediment bottom with Caulerpa sp. and small
patch reefs in left background. 44 m depth. Note the mounds produced by callianassids on sediment. Diver photograph; c, Benthic
community with "button corals" and Caulerpa algae, vertical benthic camera (55 m depth); d, Dense "button coral" community
(Cycloscris and/or Diaseris) at 56 m depth, vertical benthic camera; e. Hard substratum community with relatively barren rock
surfaces, vertical benthic camera (44 m depth); f, Hard substratum community with patch reefs with stony corals and gorgonians,
vertical benthic camera (47 m depth). [Figures from Colin, 1986.]

SUBTIDAL ENVIRONMENTS AND ECOtOGY

Soft Substrate Coverage

O >50% O >90%
Algal Flat Algal Film

C >2S% 3 >25%
Algal Flat with Algal Film

• >25% each
Button Corals

O>50-100 m-2

to o 9 "^

^N^W^TA^ ATOLC

r\j| iM

IM (M Ml

.359

256

-ll- JO'

.254

.252

ix>

o
Sl,250

|J4S

.24t,

Fig. 4 Distribution of soft substratum communities at benthic photographic station in the deep lagoon (<30 m depth), Enewetak
Atoll (Colin, 1986).

COLIN

Hard Substrate Coverage
*>20%
>50%

V lO

, _^ ; . I \fN^WETAf: _ ^ATQlI^ , _ . ,

i ' i .1

C ^ <D O ? M

! ■ i

.i l-i-i

col o

Ol -

cmI (vj

z^a

Fig. 5 Distribution of hard substratum communities at benthic photographic stations in the deep lagoon (<30 m depth),
Enewetak AtoU (Colin, 1986).

SUBTIDAL ENVIRONMENTS AND ECOLOGY

the deep lagoon by observations and photographs from the
Makali'i. Such areas appear much lighter when viewed
from above because the algal mat has not had time to
regrow. The distribution of stations with over 25% algal
mat is shown in Fig. 3.

Algal flats are dominated by macroalgae, particularly
species of Halimeda and Caulerpa. Algal films can also
occur with macroalgae. Individual plants are often
separated by areas of open sand, but densities can be high
enough that relatively little sediment bottom is visible.
Caulerpa sp. grows via rhizomes, spreading out over the
bottom in easily distinguishable patterns.

Areas of algal flat can range from a few meters to over
hundreds of square meters. On benthic photographic
traverses of large algal flats, decreasing density of thalli on
their edges was often seen grading into open sand. The
situation also exists where dense macroalgae changed
abruptly into open sand. The distribution of stations with
more than 25% algal flat is shown in Fig. 4.

The algal flat community is a diverse soft-bottom com-
munity with many epiphytic organisms with an apparently
higher biomass than open sand and algal films. A
shallow- water algal flat community, dominated by Halimeda
spp., occurs as shallow as 18 m and is a shallow lagoon
community accessible to divers, comparable to the algal
flat of deeper water. Termed "Halimeda meadows," these
algal communities extend shallower than the 30 m arbi-
trary cutoff in the "deep" lagoon, but because of strong
similarities these are considered representative of deep
lagoon Halimeda communities. These consist of extremely
dense stands of Halimeda spp., associated with other abun-
dant macroalgae, which often form distinct circular or
irregular algal communities from a few to many tens of
meters across. The meadows arc often a slightly elevated
mound, perhaps 1 to 3 m higher than surrounding sedi-
ment bottoms. Some stony corals, principally finely
branched Acropora and small head coreils, often occur on
the mound. The skeletons of these combined with
Halimeda and other calcareous algae plates produce a
higher percentage of coarse material in the surface sedi-
ment than in surrounding op)en sediment areas. The eleva-
tion of the Halimeda mound is probably due to accumula-
tion of carbonate material produced at the mound,
whereas adjacent sediment areas have not kept pace with
the relatively rapid carbonate accumulation on the mounds.
Information is lacking concerning the growth and longevity
of Halimeda mounds; such information would be of particu-
lar interest. The mounds/meadows are also foci for high
animal abundance. Small fishes, particularly herbivores and
bottom feeders, are abundant, as are benthic invertebrates.
Preliminary production/respiration data (MPRL, 1981) indi-
cated that in spite of high algal biomass there is little or no
net productivity by the Halimeda mound community, the
production of the high algal biomass being usurped imme-
diately by resident animal populations.

A community of smeJl coretls of the genera Cvchseris
and Diaseris growing unattached on sediment substrates

has been found in the deep lagoon (Colin, 1985). In this
community, the small (2 to 6 cm) corals, called "button
corals," occurred in densities of up to 100 m~ and were
photographed at only six benthic stations (Fig. 4). All these
were in the deep)er portions of the lagoon below 50 m.
The photographs at that station were all, or nearly all, of
C^icloseris- and Diaseris spp.-dominated bottom. Obviously
beds of this coral can cover areas many hundreds of
square meters. Also, macroalgae — fiarticularly species of
Halimeda and Caulerpa — small sponges, and other inver-
tebrates often occurred among the button corals.

Some elements of the fauna of soft substrata at diving
depths at Enewetak are fairly well known. In the case
where sp>ecies occurring at Enewetak are documented,
other biological information is usually not known. In one of
the few instances where more than the base essentials are
known, several species of irregular sea urchins occur
buried in, or on, sediments. The density of given sp>ecies In
apparently similar areas of the lagoon margin has been
documented to vary by well over an order of magnitude
(V. S. Frey, unpublished data). Similar population varia-
tion has also been observed at a single station over severed
months. Although these variations have been documented,
the many factors determining papulation structure of infau-
nal organisms are poorly understood.

The smaller organisms dwelling in sediment bottoms
are more jxxjrly known. For example, using a technique
where an area of bottom is covered by a plastic sheet and
rotenone, or another toxicant, introduced beneath the
sheet for a time, lancets (Branchiostomldae) have been col-
lected recently at a density of approximately 100 individu-
als m~^ on sediment bottoms below 15 m at Enewetak
(Suchanek and Colin, 1986). Schultz et al. (1952), in spite
of their collecting efforts in the Marshedl Islands, took only
a single specimen of lancet at Bikini Atoll. Approximately
50 small unidentified ghost shrimps were collected per
square meter using this technique, a density far greater
than imagined. The only visible evidence for the presence
of these small calllanassids is small-scale conical mounds
present in combination with larger mounds produced by
larger species. Also collected were stomatopods, sipuncu-
lids, molluscs, and echinoids (Suchanek and Colin, 1986).
Interestingly, in sediment-leveling experiments the number
of small-scale mounds (less than 5 cm diameter) was an
order of magnitude or more greater than large-scale
mounds, supp>orting evidence of the high populations of
small callianassids (Suchanek et al., 1986).

Pinnacle Reefs of the
Deep Lagoon

It is impossible to draw an absolute line where the
f)atch reefs on the margin of the lagoon and pinnacle reefs
begin. A working distinction can be made between "patch
reefs" which rise from a surrounding sediment or rock bot-
tom which is visible from the surface under normal condi-
tions and "pinnacle reefs" rising from depths where the

100

COLIN

surrounding bottom is not visible. Emery et al. (1954) used
the term "coral knoll" for such structures, but this author
thinks it is not truly a descriptive term in this case.

The pinnacle reefs of Enewetak Lagoon cover only a
small percent of the bottom area but are areas of great
biological diversity and interest. Their presence in the deep
lagoon, appearing as light areas among the dark waters,
parallels on a smaller scale the presence of atolls in the
deep ocean. Pinnacle reefs vary greatly in size, from a few
tens of meters to over 1 km in diameter at their base.
Emery et al. (1954) pointed out that among the 20 largest
pinnacle reefs, they are quite evenly spaced throughout
the lagoon. For several reasons the distribution of smaller
pinnacles, though, is not as well known. The tops of most
are not visible from the surface and because of their small
size, they are easily missed by echo sounding surveying.
Emery et al. (1954) estimated there were about 3000
coral pinnacles in the Enewetak Lagoon but ignored any
which did not have a relief of more than 4 m. There are
about 150 to 180 pinnacles which should be visible from
the surface (<18 m depth), rising from depths of about
35 m or more.

The surface-visible pinnacles are the best known
because they can be located relatively easily for diving and
are shallow enough for prolonged scuba diving. They have
been used as sites for a variety of studies, but their origins
and underlying structure are not well known.

The slope of the sides of pinnacle reefs can vary
greatly. In general, the smaller a pinnacle reef diameter,
the steeper its slope. On small pinnacles much of the sloF>e
is nearly vertical. The largest pinnacles are somewhat flat
on top for much of their diameter but still slope to the
lagoon floor at an angle of at least 10 to 20°.

Those pinnacles closest to Enewetak Island are best
known because of their closeness to MPRL. Figure 6 indi-
cates the location of many of these and the names applied
to them. There is, however, considerable variation in the
biological communities between pinnacles, even among
those of similar size and shape. A few pinnacle reefs are
described subsequently in greater detail.

An example of a well-developed small, but not typical,
pinnacle reef is "Pole Pinnacle," so named because of a
toppled marker pole and anchor block on its upper sur-
face. It is located 1.6 km from Jedrol Island (Fig. 6). Pole
Pinnacle actually rests on the edge of the deep channel on
an extension of the wedge of shallow reef produced by the
split of the deep channel west of Jedrol. The entire upper
surface of the pinnacle is dominated by the coral Pontes
rus, the P. iuxjyamaensis of Wells (1954) (Veron and
Pichon, 1982). On the upper surface at 3 to 5 m depth,
the columnar form of P. rus occurs, but on the sides of
the pinnacle where P. rus also dominates, the plate-
columnar form occurs. The vertical distribution of P. rus
varies on different sides of the pinnacle. On the northern
face, little occurs below 8 m, whereas on the south side a
solid cover is found above 12 m. The eastern face has its
first colonies of P. rus at about 26 m, with large patches
starting at 18 m. The western face has some large clumps
as deep as 15 m. Below the steeply sloping upper portion,

the bottom becomes less steep, having an angle of about
45° to depths of 30 m. The bottom around the base of
the pinnacle becomes relatively flat with coarse
Ha/imeda-dominated sediment and occasional small reefs.
On the eastern side, which abuts the side of the deep
channel, the bottom slopes away farther to about 40 m.

Below the depth of P. rus dominance, the coral cover
is low. The bottom is largely rocky substrate with shelves
on which considerable quantities of sediment are retained.
Hillis-Colinvaux (1980) reported that Pole Pinnacle "pos-
sessed the same high Halimeda species richness" encoun-
tered in some shallow water interisland channels. She felt
the Halimeda species populations of the sides of all pinna-
cles "may well be principal suppliers of carbonate to the
reef floor." In light of recent information on "Halimeda
meadows" and the occurrence of Halimeda in the deep
lagoon, pinnacle reefs may be less important as carbonate
producers than previously suspected, but they are still sig-
nificant. Both small and large pinnacles are definitely
Halimeda spp. sediment source points; their sloping sides
and shallow depths producing a potentially radial dispersal
of Halimeda plates from shallower depths to the deep
lagoon.

A well-known example of a "larger" pinnacle reef is
South Medren Pinnacle, located 1.7 km west of the south
end of Medren (Fig. 4). It is about 100 m in diameter,
roughly circular, and slopes off at about a 30° angle to the
lagoon floor at 35 to 40 m. Its upper surface is rugged,
with coral ridges and heads interspersed with deeper rub-
ble areas. Coral coverage is not as high as Pole Pinnacle
but seems average (10 to 30%) for most pinnacle reefs.

Coral distribution on the tops and flanks of pinnacles,
particularly larger ones, seems somewhat patchy (Fig. 7).
Definite sediment downfall areas exist on large pinnacles
which restrict corals. Medren Pinnacle has several on its
southern face, and near the base of the pinnacle at 35 to
40 m only isolated areas of reef exist. These small patch
reefs are generally of low relief, somewhat rounded with
abundant macroalgae populations. Here the large blue
tubular to vasiform sponge, Cribochalina olemda, is often
common.

Gilmartin (1966) found the green alga, Tydemania
expeditionis, along with species of Caulerpa, Halimeda,
and Dictiiota to form the bulk of algal biomass on the deep
lagoon coral patches at depths greater than 40 m. Previ-
ous dredging work on 7. expeditionis had indicated it to be
uncommon, but Gilmartin (1966) found it to be first or
second in abundance among algae on deep lagoon coral
patches, equal to or exceeded only by Halimeda at 51 to
62 m depth.

Ship Channel #1 Pinnacle (not shown in Fig. 6),
located some 6 km west of Ananij Island, is unusual. It is a
fairly small pinnacle, about 100 m in diameter, rising
within about 3 m of the surface with the lagoon about
40 m deep around it. The eastern end of its top is dom-
inated by Porites rus, similar to that found at Pole and
Tunnel Pinnacles, while its western end has almost
exclusively table Acropora corals and appears to have
been devastated by a storm several years ago.

SUBTIDAL ENVIRONMENTS AND ECOLOGY

101

2. '■

iKjj4 *

Medren

Enewetak

Fig. 6 Locations of lagoon pinnacle reefs in the southeastern portion of Enewetak Atoll. Many of these pinnacles have been
important collection localities and are not named on any other published charts. Names used are as follows: 1. Tunnel Pinnacle.
2. unnamed. 3. unnamed. 4. Cucumber Patch. 5. Dead Pinnacle, 6. Pole Pinnacle, 7. unnamed, 8. unnamed. 9. unnamed,
10. unnamed. 11. unnamed. 12. Medren pseudopinnacle. 13. south Medren Pinnacle. 14. Reefer 8, 15. Sand Island *1, 16. Sand
Island *2, 17. north Enewetak Pinnacle, 18. Marine Pier Pinnacle, 19. unnamed, 20. unnamed, 21. Harry's Patch, 22. Gemini
Pinnacle, 23. Power Plant Pinnacle. 24. Friendly Fish (Bubblebut), 25. Mini Power Plant Pinnacle, 26. unnamed, 27. Garbage Pier
Pinnacle.

Asparagopsis taxiformis is perhaps, after Halimeda
spp., the most common algae on pinnacle reefs {Fig. 7). Its
upright thalli protrude from most rocky areas, often in
dense stands. Schleck (MPRL, 1978) found that A. taxi-
formis grew in a band about 1 m wide and several hun-
dred meters in length along leeward island lagoon shores.
In deeper water, Schleck reported it formed an abundant.

but scattered, community with a vertical distribution to at
least 20 to 30 m.

The Lagoon Margin

The lagoon between 30 m depth and
islands or the reef flat, the "lagoon margin,"

the shore of
is an area of

102

COLIN

Views of Lagoon pinnacle reefs. Upper left and lower

left: Coral development (Pontes rus) on the western side of "Tunnel Pinna-
cle" (Fig. 6) with extensive development of the plate-like growth form of
this cora! from about 5 to 18 m depth. Upper right: Typical view of
lagoon pinnacle (Tunnel Pinnacle) at about 25 m depth with the coral
Pauona cactus and the sponge Cribochalina olemda visible. Much of the
substratum is devoid of cora) and has an algal community growing on the
rock surfaces. Lower right: The algae Asparagopsis taxiformis which is
abundant on most lagoon pinnacle and margin patch reefs.

great transition. The width of the lagoon nriargin varies
considerably from only a few hundred meters at the south
end of Enewetak Island to over 1 km from Lojwa north to
Engebi and Boken. The deeper portions are similar to the
deep lagoon, and because of their accessibility to scuba
divers, are an excellent area for studies relevant to the
deep lagoon. From about 6 m to 15 to 20 m depth, the
bottom has areas of relatively steep sediment slopes, often
at the angle of repose; abundant patch reefs, often with

relatively high vertical relief, high coral diversity, and abun-
dant fish populations.

The windward lagoon margin is strongly influenced by
the reef flat. Areas of high water transport across the reef
flat ("rips"), found at the ends of islands and also along
interisland reef flats, affect the distribution of sediments
and patch reefs on the lagoon margin. In the lee of the
large islands (Enewetak, Medren, Runit) patch reefs are
somewhat "dead," with relatively low coverage of corals.

SUBTIDAL ENVIRONMENTS AND ECOLOGY

103

A different situation exists on the lagoon margin on the
southwestern, western, and northwestern sides (leeward).
Because of exposure to prevailing winds across the fetch
of the lagoon, these areas often possess an almost barrier-
reef type structure with small patch reefs inside it. The
sediment bottom often slopes upward steeply near this
structure. This is discussed subsequently.

Hiatt and Strasburg (1960), in their classic study of
reef fish feeding ecology, presented a brief summary of
Enewetak reefs. They reported that in the lagoon "in pro-
tected areas there is a discontinuous series of irregular
patch reefs which extend from nearshore to the outer reef
slope leading to the deeper parts of the lagoon." On the
western side of the lagoon, "the lagoon reefs are better
developed and frequently are continuous, because they
receive fairly strong waves engendered by the prevailing
winds" across the lagoon. In some respects, they come to
resemble reefs of the windward shore. Hiatt and Strasburg
(1960) provide drawings of typical reef environments (tidal
pools, seaward reef flat, spur and groove surf zone, patch
reefs and coral heads, mid-water) with the characteristic
fishes found there.

The patch reefs of the windward lagoon margin have
particularly well-developed coral communities where the
water flow across the reef is unimpeded by islands. The
vertical relief of the reef generally increases with size, but
in many cases small reefs have a relief about one-half their
diameter, up to a maximum of about 6 m relief. Table
Acropora sp. corals are abundant on these patch reefs,
whereas other corals grow well on the sides of the patch
reefs and even under overhangs because of the reflection
of light from the white bottom. Relatively few soft corals
occur in such areas.

Sand areas in between the lagoon rim patch reefs are
areas of high grazing pressure by surgeonfishes and parrot
fishes. Burrowing activity in the sediments is also high,
mainly through the activities of a variety of fishes.

An important factor determining the distribution of
windward lagoon margin patch reefs is the effect of lagoon-
ward sediment and rubble movement from the reef flat.
Between Enewetak and Medren such patches are abun-
dant, but they are best developed in areas protected from
sediment "overwash." Leeward of Bokandretok is an area
of numerous patch reefs, whereas north and south of this
the island rips have covered the area with sediment where
the reefs occur. Farther north along the reef, areas of sedi-
ment overwash have at best reduced numbers of patch
reefs. In areas protected by structures diverting the cross-
reef flow of sediment, patch reefs are better developed,
coming close in behind the reef flat. Nolan (1975) used a
large series of patch reefs in the lee of "Isaac's Island," a
small rock and sand spit, for his fish community studies.

Nolan (1975) described some patch reefs between
Medren and Enewetak Islands where he analyzed and
manipulated reef fish populations on these and artificial
reefs. He felt coral development was particularly luxurious
on the patch reefs on the lagoon side of Isaac's Island.
Nolan (1975) pointed out that many of the patch reefs to

leeward of Enewetak and Medren Island were predom-
inantly dead coral. He provided a detailed map locating his
study reefs and chose reefs of about 3 X 3 X 3 m in
size, which were abundant, in depths of 5 to 7 m. He
noted that the reefs in the lee of Isaac's Island were pro-
tected from the full brunt of the cross-reef currents but
that an eddy pattern existed on the leeward side of this
small outcropping which provided abundant water circula-
tion.

Nolan's (1975) study reefs were predominated by mas-
sive "table" Acropora cythera, but during his study in
1972, heavy surge from the leeward side of the atoll
dislodged many of these corals on his study reefs. Sand in
this area was also removed and deposited in shallow water
creating a 3 m high sand bar continuous from Medren to
Enewetak. This sand ridge was destroyed and moved into
the lagoon with the resumption of normal trade wind
weather and sea swell.

Similar destruction of A c\^thera on patch reefs was
observed during southwesterly to westerly storms in March
1981 and July 1982. The tables of A. cythera were bro-
ken loose at their bases and moved. Many specimens
ended up on island beaches with the corallum nearly
intact, testament to the strength of this form.

North from Japtan to Ananij, no significant lagoon mar-
gin patch reefs exist between islands. The bottom slopes
relatively steeply into the lagoon, and the reef from ocean
to lagoon is narrow. The zonation across the reef is dis-
tinct (Fig. 10) and is described subsequently. Chinimi, the
only island interrupting this 4 km stretch of open wind-
ward reef, has the lagoon margin protected from reef flat
"outwash," and patch reefs are well developed in the lee
of the island. The change in zonation of the lagoon margin
is really visible north from Japtan. Island rips occur north
and south of Chinimi and lagoonward depth contours veer
close to Chinimi 's shore in its lee. This cusping of the atoll
rim behind islands is seen in other areas of the windward
side. The area on the northern lagoon margin of Chinimi
has one of the best developed reefs along the shore of any
windward island, with lovely microatolls, although less than
100 m north the reef seems limited by the island rip and
sediment outwash.

Ananij similarly has a large number of lagoon margin
patch reefs in its lee and has the most developed island rip
system of any island at Enewetak. Between it and Runit,
8 km farther north, cross reef zonation is similar to that
south of Ananij, but more islands are found on the reef.
The island cusping effect, however, is evident with many
patch reefs in their lee.

A good example of a well-developed lagoon margin
patch reef is "Choptop Reef," located just north of "Isaac's
Island" between Enewetak and Medren (Fig. 8). It is large
for a lagoon margin patch reef, but smaller reefs adjacent
to it are similar and provide easy comparison. Choptop
has high coral cover and diversity and high fish popula-
tions (Fig. 8). It is located on the margin of a reef flat rip,
and although not in the strongest portion of the current
coming off the reef flat, it is in a well-flushed area. An

104

COLIN

Fig. 8 A lagoon margin patch reef. "Choptop Reef," from the air. The main reef (center of photograph) is surrounded by smzdier
"Satellite Reefs," some of which are Pontes cylindrica colonies probably broken from the main reef by storm waves. The reef flat
is seen in the upper left with a sediment/rubble bar, produced by a cross reef "rip" seen in the upper center. Water depth around
Choptop Reef is about 6 m. Upper right: Typical view of coral development on a lagoon margin patch reef (Choptop ReeO with
the sediment floor surrounding the reef visible in the background. Lower left: View of upper surface of a lagoon margin patch reef
(Choptop Reef) with abundant coral and fishes visible. Depth on the top of the reef is approximately 2 m. Lower right: "Satellite
Reer' located about 15 m away from the main portion of Choptop Reef. This reef is simply a smaller version of Choptop with a
vertical relief of about 4 m.

aerial photograph of the reef is shown in Fig. 8, with the
rubble bar and outwash area of the reef flat rip clearly visi-
ble.

There are several smaller "satellite" reefs close to
Choptop which may have resulted from storm fragmenta-
tion of the larger reef (Fig. 9). The sediment around
lagoon margin patch reefs, like Choptop, is coarse. Cal-
careous macroalgae, such as Halimeda spp., occur
sporadically on the lagoon margin (Fig. 9), not in large
beds as is found in the deeper lagoon.

Coral heads on the upper surface of lagoon margin
patch reefs often rise to near the surface, but at Enewetak,
patch reefs are not planar at about mean to low water
levels. At Ujilang Atoll, 200 km southwest, lagoon patch
reefs were planar on top, reaching low water level,
because of growth of coralline algae. Enewetak patch reefs
lack abundant coralline algae on the upper surfaces which

may account for these differences. Encrusting corallines are
abundant within interstices of Enewetak patch reefs, but
the difference, compared to Ujilang, in the amount of
exposed corallines is striking.

Where the internal structure of patch reefs is exposed,
such as in caves or recent fractures, it appears to be com-
posed of accumulations of coral skeletons that are poorly
cemented internally. Dead branches of coral plates have
interstices where small sclerosponges are common. Smith
(MPRL, 1972) reported that an explosive blast on a lagoon
pinnacle west of Jedrol "exposed unconsolidated to poorly
consolidated coral material more or less in growth posi-
tion." Sclerosponges, one of the prominent inhabitants of
the unlighted holes in the reef, were abundant.

The sediment in these lagoon rim areas is not neces-
sarily stable. At some coral patches, the sediment is
scoured away at the base of the patch reef. Likewise, in

SUBTIDAL ENVIRONMENTS AND ECOLOGY

105

Fig. 9 Environments of lagoon margin patch reefs. Upper left: Coarse carbonate sand bottom with Hallmeda spp. and other
macroalgae. depth 6 m, near Choptop Reef. Upper right: Sediment/rubble bar near Choptop Reef produced by cross reef "rip."
Lower left: "Satellite Reef near Choptop Reef, comprised of a single colony of Porites cylindrica, probably torn from the main reef
by storms. Several other satellite reefs are visible in the background. Depth on the bottom is 6 m. Lower right: Small patch reefs
on the lagoon margin. A large cable from the atomic testing period is draped over a small patch reef (indicating an age of at least
20 to 30 years) with a colony of Porites edouxi;i which has grown on the cable, depth 5 m.

some areas sand can be piled against the reef-killing corals
or other sessile invertebrates. Coral colonies with half their
surface buried and dead and the upper half healthy can be
found at the point of reef-sediment contact on some patch
reefs. Alteration of normal tradewind sea conditions can
radically alter shallow water sediment distributions.
Beaches grow or recede, islands change, and shallow sand
bars on the lagoon margin appear or vanish with changes
produced by passage of cyclonic storms (Nolan, 1975). It
is not necessary for storms to pass close to Enewetak
because the swell produced by a distant storm can accom-
plish the listed changes without high winds.

Lagoon Margin Zonation

The area immediately lagoonward of the reef flat is
quite variable and of considerable biological interest. Vari-

ous authors have described this zone, usually in combina-
tion with a description of a cross-reef flat transect.

Odum and Odum (1955) described the zonation of the
interisland reef about 400 m north of Japtan Island. In
many respects this is typical of windward interisland reefs
of the central and southeastern portions of Enewetak.
They described six zones from ocean reef to lagoon (a dis-
tance of about 450 m). These were (1) a buttress zone,
(2) the algal-ridge, (3) an encrusting zone, (4) a zone of
small coral heads, (5) a zone of small patch reefs "larger
heads," and (6) a sand and shingle zone. Typical views of
the bottom on the Odum and Odum (1955) transect are
shown in Figs. 10 and 11. They make the point that the
interisland reefs generally had more "vigorous" communi-
ties as opposed to reefs seaward of islands ("island reefs")
where living corals were limited to the outmost portions of
the reef. They believed this was due largely to different

106

COLIN

Fig. 10 Upper: Aerial view of the windward reef in the area of the Odum and Odum (1955) transect north of Japtan Island. The
ocean is to the right and the lagoon to the left. The reef flat and associated reefs lie in the middle of the photograph. The photo-
graph was taken while flying over the island of Japtan: Chinimi Island is visible with Ananij Island behind It. A normid surf is
breaking on the windward reef with the lagoon margin very calm. Lower left: View of the zone of small coral heads on the Odum
and Odum transect, depth approximately 1 m. Lower right: Junction of large coral head zone with the sand shingle zone of the
Odum and Odum transect.

water circulation patterns. Johannes and Gerber (1974)
illustrated a simplified cross section of reef near the tran-
sect of Odum and Odum (1955).

In the Odum and Odum (1955) study area, the bottom
slopes gradually lagoonward from the encrusting zone. Indi-
vidual coral colonies grow upward to a level limited by low
water. In some corals the central portion of the colonies
are dead with the sides continuing to thrive, producing
structures known as "microatolls" (Fig. 12). These have
been examined further on Enewetak reefs by Highsmith
(1979) and will be commented on later. Often a distinct
lagoonward edge to the reef flat pavement exists, and in
many places, water flowing across the reef flat has eroded
away and undercut the sediment beneath this lagoonward
edge (Fig. 12). This has caused the reef flat pavement to
collapse or buckle in places. This is most evident in areas
where reef flat rips pass the edge of the pavement. The
swift currents combined with the effects of dropping off
the pavement have scoured deep potholes (as deep as 4 to
5 m) down into the sediments. The pavement is usually
undercut on these edges.

The shallow reefs of the northern lagoon are p>oorly
known. From Engebi west to Bokoluo, the reef Is broad, as
much as 1 to 1.5 km across, unlike southern reefs. Its

zonatlon can be seen In aerial photographs but has not
been investigated In detail. There Is a reef flat about 100
m wide, then a broad (to 1 km) shallow area with coral
heads. This coral head area on the west side of Engebi
was examined. There were large microatolls of Pontes
lutea and acroporld corals on a level sandy bottom.

To the west of Bokoluo lies the open expanse of the
northwest reef tract. It runs fairly straight to the northwest
corner of the atoll at the West Spit. The gentle arc of the
northwest reef is about 1.5 to 1.7 km across from the
ocean to the deepening lagoon. From aerial photographs
there appear to be four major zones: (1) a reef flat, (2) a
coral head zone, (3) a clustered coral head zone, and (4) a
patch reef zone. The reef flat Is estimated to be about
150 m across, merging with a deeper coral head zone
toward the lagoon. The coral head zone appears about
800 m across and Is complex, with three visible com-
ponents to It. The middle one-third of the coral head zone
appears deepest, whereas the lagoonward one-third
appears shallow. The density of coral heads in this area Is
high. Density data from photographs Indicate there are at
least 500,000 coral heads in this "coral head zone"
between Bokoluo and the West Spit. There is scarcely any
open sand of more than a few tens of meters between any

SUBTIDAL ENVIRONMENTS AND ECOLOGY

107

Fig. 11 Views of tiic sand-shingle zone of tfie Odum and Odum (1955) transect at Enewetak Atoll. Upper left: Coarse carbonate
rubble and sand immediately behind the large coral head zone (depth 1.5 m). Upper right: General view of rubble area behind the
large coral zone. Lower left: Carbonate sand farther lagoonward from the large coral head zone, depth 3 m. Lower right: Break in
slope of sand-shingle zone where the slope increases considerably (to the right) toward the deep lagoon.

of them. From aerial photographs it appears many of the
coral heads are arranged in a serial fashion across the reef
with large numbers of them resembling striations across
the bottom.

The clustered coral head zone is about 600 m across
and has a lower density of coral heads than the previous
zone. Those present are grouped together somewhat.
Finally there is a zone of large patch reefs about 400 m
wide. These patch reefs appear comparable in size to the
larger patch reefs of the windward lagoon margin.

Channels Between Northern Islands

The channels between the closely spaced northern
islands are of special beauty and biological interest. They
are not true passes from ocean to lagoon because they
draw their flow from the shallow reef flats to seaward but
channelize the flow of water off the reef flat between

islands. Viewed from the air, their bottom features show
strong orientation to the current which funnels between the
islands from ocean to lagoon, with reefs often elongated
with the current and sediment washed out between patch
reefs. These "passes" have a reef flat on their seaward
end, but the cross-reef flat flow from an area of reef front
several times broader than the channel is funneled into
each one. The channels are often deep, but where current
flow slows on their lagoonward end, they usually have a
shallow, delta-like bottom.

A good example of a northern island channel is that
between Lojwa and Aomen. At very low tides water flow
across the reef flat is completely eliminated, with no
current in the channel. At high tides with strong waves
pumping, the current is swift, sufficient to deeply churn
sediment from around patch reefs in the channel. The gaps
between reefs have the sediment scoured away, appearing
darker blue when viewed from above, whereas areas on
the sheltered, downcurrent side of the patch reef have

108

COLIN

Fig. 12 Upper left: MIcroatolls (Porites lobata) at the north end of Chinimi Island, Enewetak. At low tide the water Is essentially
at the upper level of the microatolls. Secondary growth is also occurring In the central area of the top of the mlcroatoUs. Upper
right: Aerial view of lagoonward edge of the reef flat showing erosion at the end of the reef pavement caused by water flowing
across the reef flat. Lower left: Typical views of patch reefs in the Lojwa-Aomon interisland channel. Extensive sculpturing of the
sediment bottom is caused by currents which course through this channel at high tide. Lower right: Area of the Lojwa-Aomon
channel with sand built up behind (down current side of) a large patch reef.

white sand built up. The width of these "tails" of sediment
decreases downcurrent of the reef. The upcurrent sides of
the patch reefs have the sand washed away to depths
equal to those on the sides of the reef. Corals and benthic
invertebrates are usually well developed on the upcurrent
end and sides of reefs.

The deepest portions of the channel are 6 to 7 m, and
some patch reefs are emergent at low tide (Fig. 12). The
reefs in this channel have changed little in the last 32
years based on aerial photographs taken in 1949 and
1981. The major patch reefs are identifiable, but some of
the lagoonward patch reefs seem to have been somewhat
buried by sediment.

Other interisland channels are similar. Rock surfaces
are heavily grazed by herbivorous fishes. Small caves and
overhangs off the patch reefs are lined with encrusting
coralline algae. These patch reefs are one of the few
places within the lagoon where branching coralline algae

are found. Sediments are coarse, with predominantly large
foram tests, coral, and Halimeda bits. The reefs of the
channel between Lojwa and Alembel seem to have been
devastated by a storm during the last decade. Very little
live coral and few benthic invertebrates arc on them. Allen
(1972) used this channel as a primary study site for his
anemonefish work. One patch reef in the channel had
more than 75 clusters of 10 to 30 individuals of
Ph^isobrachia douglasi, with larger numbers of Amphiprion
melanopus. in an area of only 700 m^. In the summer of
1981, this area was re-examined for anemones and
Amphiprion, no anemones or anemonefishes of any type
were found.

In channels farther north, corals and other inver-
tebrates seem healthy. Some of the channels were noted
for their abundance of large tridacnid clams, but many of
these clams have been eliminated since the repatriation of
the Enewetak people.

SUBTIDAL ENVIRONMENTS AND ECOLOGY

109

Passes

There are three passes from ocean to lagoon with suffi-
cient water depth for boats to regularly traverse them.
They are the "deep passage" (east) between Medren and
Japtan, the "wide passage" (south) between Enewetak and
Igurin, and the "southwest passage" between Kidrenen
(south) and Biken. Various details of these passes have
been discussed in Chapter 3, this volume.

The biological communities of the deep channel and its
margins have not been well described. Hobson and Chess
(1978) discussed the patch reefs and plankton communities
in the area between Japtan and Jedrol Islands which are
affected by currents coursing through the deep channel,
but their study site was not in the deep channel proper.

The northern side of the deep channel slop>es steeply
from depths of only a few meters. To the east of Jedrol
there is actually a "barrier" reef awash at low water which
is constantly exposed to oceanic swells entering the lagoon
through the deep channel. The northern slope of the deep
channel to depths of 30 to 40 m is a nearly 45° angle
rocky slope with abundant corals and reef-associated inver-
tebrates. At depths of 25 to 40 m, the bottom levels and
the central portions of the channel are probably relatively
flat. There is a downslope sediment transport along this
face, and below 30 m where the bottom begins to level,
sediments also begin to dominate the bottom compared to
exposed rock outcrops.

The easternmost extension of the shallow wedge where
the channel splits is distinct, the "cutting edge" being only
a few meters wide and descending at about a 45^ angle
from 6 m to depths below 30 m. The coral communities of
the shallow reef and slope are rich. The fish communities
of the north side of the channel are diverse and abundant
with zooplanktivores more dominant than in other areas.

The south side of the deep channel is different from
the north, with the bottom sloping gradually as a sediment
slope with little or no exposed rocky substratum. A shelf
between 30 and 36 m in depth extends a kilometer or
more northwest from Medren into the lagoon.

Little is known about the area of the wide channel.
Aerial photographs show large patch reefs on a sandy
bottom scattered across the entire 9.3 km width. The crest
and outer slope of the sill was examined about 1.6 km
west of Enewetak and had large, rocky patch reefs, not
unlike large lagoon margin patch reefs at 18 to 20 m
depth (Fig. 13). The patch reefs had relatively little live
coral but had abundant Halimeda spp. and Asparagopsis
taxiformis. The most common corals were Pocilhpora spp.
The sediment was coarse, dominated by Halimeda. with
small ripples at 22 m depth. There were small rocks
between the much larger reefs but little grew on them. To
seaward, the sediment bottom sloped perceptively. At
30 m, it was nearly all sediments with only a few rock
patches and sloped at an angle of about 15° (Fig. 13).
Below that depth, the slope increased to about 20° at
40 m and more with increasing depth.

The southeast passage consists of sandy channels
between elevated fingers of reef for 6.5 km southeast of

Biken. Atkinson et al. (1981) estimated the cross-sectional
area of the southwest passage as only 26% of the deep
passage and 6% of the wide passage with no net inflow or
outflow. The reef fingers have well-developed coral com-
munities which do not differ greatly from the interisland

Fig. 13 Views of the bottom, wiae ^soutn; passage,
Enewetak Atoll. Upper: Rubble substratum at about 20 m
depth looking downslope. Middle: Juncture of rubble and sand
substratum at 30 m depth, looking downslope. Lower Sand
slope substratum with isolated coral l>oulders at 40 m depth.
There is considerable evidence of downslope transport of sedi-
ment in this view.

110

COLIN

patch reefs fin the windward side. The sand channels shoal
gradually from the lagoon to their shallowest fxsint, then
again gradually deepen to seaward. Near the precipitous
reef edge to seaward, the channels quickly steepen, then
plunge down the near vertical slope. Sediment is trans-
ported over the drop-off here with heavy scouring of the
reef face below the sand chutes.

Algal Ridge

Before considering the true seaward reefs, it is
worthwhile to mention the zone marginal to the reef flat.
This is the "algal ridge" which is truly intertidal but has
extreme relevance to subtidal seaward areas.

The seaward reef on the windward side of Enewetak is
mostly devoid of live coralline algal ridges. Live algal ridge
(often termed "Lithothamnion ridge" by earlier authors)
occurs only along one section of windward reef about
200 m in length at Ananij Island. This section is readily
distinguished by its pink coloration, produced by the abun-
dance of Porolithon species, as compared to the dull sur-
face of the algal ridge dominated by macroalgae.

Three species of Porolithon, as identified by Lee
(1967), have been found on the Ananij algal ridge. Large
portions of the surfaces of the spurs are covered with crus-
tose corallines, probably Porolithon onkodes. Distinct colo-
nies of Porolithon craspedium, often with blunt fingers
forming a lobate mass, occur scattered on the upper sur-
face of the spur. Porolithon gardineri seems the least com-
mon species, although its colonies are often irregular
masses 20 cm or more across. It appears limited to the
sides of the spurs, not being found on the upper surface
among P craspedium^ Within the sponge-like structure of
the spurs at Ananij, virtually all visible internal surfaces are
covered by coralline algae, but the species involved are not
known.

Inshore from the live algal ridge at Ananij is a slight
depression of the reef flat where colonies of Acropora sp.
flourish. Small patch reefs occur on the hard pavement
here which has water on it even at low tides. The
Acropora sp. colonies are emergent at low tides. The small
coralline algae Neogoniolithon rutescens is occasionally
found among these patch reefs but not on the more
exposed spur and groove areas.

Seaward Reefs

Smith and Harrison (1977) described the windward
reef slope off Chinimi Island, and since their study other
areas have been examined. The spurs are relatively flat on
top and occasionally have undercut, overhanging edges
(Fig. 14). Algae and invertebrates are abundant on the
sides of these spurs. Sea urchins have eroded elongate
grooves in the rocks on the sides of the spurs which afford
protection from wave action and predatory fishes (Fig. 15).

The bases of the grooves are floored with boulders and
cobbles, precluding any significant benthic invertebrate
populations (Fig. 14). The walls of the grooves, however.

have on them small corals and invertebrates adapted to
withstand the wave surge. On the upper surfaces of the
spurs, small corals grow with an abundant film of algae on
rock surfaces (Fig. 14).

Smith (MPRL, 1972) dissected a spur and groove sys-
tem north of Japtan using explosives. "The spur proved to
be dense, well-cemented coral rubbles covered by a veneer
of live encrusting coralline algae." He felt that, except for
relatively minor growth by the coralline algae, the spur and
groove systems are erosional features.

On windward reefs the spur and groove zone and the
area immediately seaward of it are areas of very high fish
abundance (Fig. 15). Herbivorous parrot fishes and sur-
geonfishes feed in this productive area and range on to the
algal ridge and reef flat from there. At low tide these shal-
lower areas are dry, requiring their exploiting fish popula-
tions to move elsewhere.

The spur and groove zone seaward of the areas of
Porolithon algal ridge at Ananij is different from other
areas examined where the ridge is "dead." The cover of
benthic invertebrates appears higher there. This is the only
area on the windward shore where the club-spined urchin
Heterocentrodus trigonarius is known to be abundant, both
in holes on the sides of the spurs and on the algal ridge. A
form of branched Acropora sp. coral with other corals and
Halimeda sp. algae with distinct laminations occurs there.
This form of Acropora has not been seen elsewhere
(Fig. 15).

Off the north end of Enewetak Island, the sides of the i
spurs are lined with grazed macroalgae and occasional
patches of coralline algae. The rock-boring urchin,
Echinometra methaei. is abundant in grooves in the sides
of the spurs. In small caves and on overhangs a wide
variety of benthic invertebrates occurs.

On the sides of the spurs' upper surface are small head
corals and soft corals. There is less coral on the tops of
the spurs, and the area is more dominated by macroalgae. |
At the seaward end of the spur, colonies of stony corals,
Heliopora caerulea, and soft corals are common. These are
larger than those of the top or sides of the spur. Some
sizeable encrusting sponges may also occur in this area.

Several herbivorous fishes are characteristic of this j
spur and groove zone. The surgeonfishes, Acanthurus
achilles. A. guttatus, A. thostegus, and especially A. lin-
eatus are generally found in any abundance only in this
area on the windward shore. One small damselfish,
Plectrogliiphidodon phoenixensis, is common on the wind-
ward shore and occurs only in the spur and groove area.

Seaward of the spur and groove, the rocky bottom lev-
els somewhat with only a slight seaward slope (Fig. 14).
The bottom often has minor undulations of its surface,
occasionally with small shallow grooves oriented perpendic-
ular to the reef front, but generally it has few distinguish-
ing features. The irrejular pits and grooves of rock-boring
sea urchins, Echinometra mathaei. and lesser numbers of
some diademnid urchins (Fig. 16) are often abundant. A
few small- to medium-sized corals occasionally occur on i
this "barren" zone (Fig. 15). Viewed from the air, this zone -

SUBTIDAL ENVIRONMENTS AND ECOLOGY

111

Fig. 14 Typical views of the spur and groove zone off Enewetal( Island. Upper left: Shallow groove with large coral boulders In
its center which are set in motion during periods of high waves. These effectively keep the grooves free of sessile benthic macroor-
ganisms. Upper right: A variety of herbivorous fishes at seaward end of a spur. Lower left: Spur seaward of the Enewetak Island
reef flat with breakers rolling over it. Lower right: Seaward end of a spur with a breaker forming at its prow. The shallower water
depth on the spurs causes waves to break there sooner than over the grooves.

appears to be a uniform light color. The rock shelf width
varies around the atoll. Off Enewetak Island it is relatively
wide, about 200 to 300 m, but farther north it becomes
narrower, probably less than 100 m wide.

The rock surface of the shelf often has evidence of
extensive boring by clionid sponges. Large areas of
substrate may have the tiny, dark oscula visible, but these
are not apparent on superficial examination (Fig. 16).

Smith and Harrison (1977) have described a windward
reef slope from off Jinimi Island in connection with
estimates of calcium carbonate production there. The reef
crest had essentially no corals. Moving seaward from the
reef crest and spur and groove zone, the bottom slopes
gradually from 4 to 5 m depth to about 8 m and is essen-
tially a rocky pavement with minor surface undulations.
Smith and Harrison (1977) estimated only 10% coral cov-
erage in their study area at 7 m depth. Seaward, the
amount of coral cover increased with depth, although the
slope may increase only slightly with 15, 20, and 25% at

11, 15, and 21 m depth, respectively. At 50 m, coral cov-
erage was virtually zero. Smith and Harrison (1977) found
that the vasiform Acropora cvthera was the most conspicu-
ous coral in their study area, with its nearly flat upper sur-
face well adapted for capturing sunlight. They performed
coral and coralline algae incubations using clear acrylic
domes, where possible, at depths to 21 m. Steadily
decreasing rates of calcification with increasing depth were
found. Overall they believed the seaward slope of wind-
ward reefs at Enewetak (the "mare incognition" of Ladd,
1961) has only a small role in the CaCOa mass balance of
the atoll.

Large numbers of vasiform Acropora cilthera colonies,
up to 2 m in diameter, were found by Smith and Harrison
(1977) at 15 to 25 m at their study area (Fig. lA of that
paper). Colonies had a maximum of 13 growth bands
(annual), and they considered that the major typhoon in
late 1962 (their observations were in late 1976) may have
devastated Acropora corals in that area. Smith and

112

COLIN

Fig. 15 Upper leh: Larger shoal of Acanthurus triostegus in the spur and groove zone, Ananij Island, depth 4 m. Upper right
Unusual growth form of Acropora sp. found seaward of the area of live edgal ridge, Ananij Island, Enewetak Atoll. Lower left:
Grooves eroded in the side of spurs by sea urchin Echinometra mathaei, windward reefs, Enewetak Atoll. Lower right: Isolated
coral head located to seaward of the spur and groove zone, windward shore of EnewetiU( Island, depth 7 m.

Harrison's (1977) study area was disrupted by a severe
typhoon in January 1979 (Alice) in which all the large
Acropora colonics at 15 to 25 m were reduced to rubble
(Fig. 16), confirming their suspicion that typhoon-strength
storms are capable of such disruption to depths near 20 to
25 m.

The outer slope or "drof>-off" begins at depths of 18 to
23 m as a distinct change, from a gentle slope of a few
degrees to an angle of approximately 30° to 45°. This
slope rapidly increases with depth (Fig. 17). The deep
reefs of the windward side have been severely damaged by
storms so that there is relatively little live coral and
tremendous amounts of rubble at 15 to 30 m depth
(Fig. 16). Along Enewetak Island to Medren, there is gen-
erally a sandy zone at 30 to 40 m which appears as an
irregular light band from the air. Below this depth sand
channels alternating with reef can be seen on the outer
slope when viewed from the air; this sandy zone is not
apparent from the air on reefs of the islands farther north.

Vosburgh (1977) experimentally determined that waves
of near 5 m height did not produce sufficient water motion
at depths of 9 to 21 m to cause breakage of the skeleton
of large, healthy Acropora Ci/thera. He reported that
although this species is found at less than 2 m depth in
sheltered areas of the lagoon, it occurs commonly on the
windward reefs only at depths below 8 to 10 m. Sheltered
lagoon colonies were generally larger than those on the
windward reef, and depth distribution and colony size are
related to wave exposure. Although his estimates of near
5 m waves are based on the highest 1% of waves
observed during the windiest portion of the year, he points
out that typhoon waves, not considered in his study,
"might cause catastrophic breakage over the entire species
range on the (windward) terrace."

The steady seaward slope of the windward reef gen-
erally prevents accumulation of large amounts of sedimen-
tary material. At the slope break at about 18 to 20 m
depth, some sediment-bottomed channels occur which can

SUBTIDAL ENVIRONMENTS AND ECOLOGY

113

'-^u

■W
> •,.

"•c

' . ■+'■ ;

^ ' ■

J^ft"

*-■-'.

*/'

%•

", '!_' ^ ■

, V

r 1

:, i f

/,--.^

SsAi.;*. , >i' ' ;^'

m^y^ut- W.

Fig. 16 Upper: Carbonate rock substratum heavily bored by
the sponge Cliona sp. on the windward reef, Enewet€U< Island,
depth 8 m. The dark oscula of the sponges are visible over
much of the substratum, although the tissue of the sponge is
located internally beneath the surface of the rock. Middle:
The area of the shelf edge break (20 m depth) off Enewetak
Island. There is very little live coral in this area with only a
single table Acropora visible among large amounts of coral
rubble. Lower: Outer slope at 25 m depth. Enewetak Island,
coral rubble going down the slope into deeper water.

serve to transport sediment into deep)er water. Below the
shelf break, larger amounts of sediment are visible on
relatively horizontal areas, but the slope limits the amount
of build up.

The reefs of the leeward side have extremely steep
slopes. The distance between the reef crest and the steep
slope into deep water changes with location. Along the
southwestern islands (Ikurin through Kidrenen) there is a
narrow shelf sloping gently from about 3 to 15 to 18 m.
This shelf is generally about 100 to 150 m wide and has a
well-developed coral community on the rocky shelf. Most
of the corals are small, less than 10 years old, implying
recent devastation, probably by storm waves. Sand chan-
nels occur perpendicular to the reef front which is at the
head of reentrants on the reef face. The change to a steep
slope occurs at about 15 m where it becomes a 45° to
60° slope to the limit of scuba diving. A typical profile of
a southwest island reef is shown in Fig. 17.

To the west of Kidrenen, the reef remains unbroken
until the southwest passage. The bottom slopes gently,
then progressively becomes steeper with virtually no shelf
to a near-vertical face at about 10 m depth. The horizontal
distance from water a few meters deep to the vertical face
is less than 50 m. This extremely steep profile is even
more pronounced on the reef north of Biken to the West
Spit. Reentrants penetrate the reef face with Halimeda
dominated sediments on shelves on a steep slope into the
deep water (Fig. 17).

The leeward reef crest near the island of Ikuren has a
healthy cover of coralline algae on its upper surface, even
though on the leeward side of the atoll, small to moderate
surf usually occurs, which is produced by the long, low
swell from the west. Large numbers of herbivorous fishes
occur here, essentially the same species as are found on
the windward spur and groove areas. The two areas are
similar; but near the southwest islands the grooves,
strength of surf, and various invertebrates are lesser
developed. Seaward of the reef flat are often small high
relief rocky structures with flattened tops and abundant
coral (Fig. 18). Species of Acropora, Pocillopora, and
Heliopora axe common on the edges of the coralline flat.
The cidaroid sea urchin, Heterocentrotus trigor)arius, is
found deep in small caves and crevices of the outlying rock
structures among coralline-covered fossil coral branches.
Around and to seaward of these structures is often a bot-
tom at 5 m depth composed of large coral boulders and
shingle. Much of the hard substrate in this area not
covered by hard or soft corals has coralline algae growing
on it. These algae often have large numbers of grazing
marks almost cerlainly from parrot fish (Fig. 18). The alga,
Asparagopsts taxiformis, is extremely abundant; its upright
tufts in evidence on nearly all rocky surfaces (Figs. 19 and
20).

A rock substrate begins within 20 to 40 m of the reef
flat with occasional large vertical knobs of rock covered
with hard and soft corals. Urchin grooves are evident in
the rock, but diademnid urchins were seen much more
often in them than Echinometra mathaei.

114

COLIN

30 60

120

DISTANCE, m

180 240 300

360

420

5X Vertical Exaggeration

Fig. 17 Typical slope profiles of Enewetak Atoll seaward reefs. The profiles, which are vertically exaggerated, are from a wind-
ward reef off Enewetak Island (upper), a leeward reef off Ikuren (southwest islands) (middle), and a leeward reef north of Biken
(lower). Dotted lines represent the bottom in areas of sand channel reentrants of the reef face. Waves shown on the surface reflect
the normal wave conditions on these different areas.

The rocky shelf slopes gradually seaward, and at about
8 to 10 m depth sand channels begin to appear on its sur-
face. There is considerable relief between the reef fingers
at about 9 to 10 m and the channels at 12 to 14 m (Figs.
19 and 20). The sides of many of the fingers are nearly
vertical and often undercut. These overhanging walls have
dense coverage of coralline algae and abundant Haliweda.
The sediment in the channels is coarse, derived largely
from Halimeda flakes and often has wave ripples on its sur-
face from the long period swells. The upper portions of
the reef fingers have dense coral on their tops and sides.
Coral coverage at 12 m depth on the top of the fingers at
the shelf break is 80 to 90% in some areas. A few large
head corals occur but most are small to medium acro-
porids. They are at most 25 to 40 cm across and probably
reflect recruits after storm destruction of most of the previ-
ous acroporids (Fig. 19).

The bottom slopes away at the shelf break (12 to
15 m) at a 45 to 60° angle. Most of the sand channels
continue down the slope as sediment chutes into deep
water. These chutes are cut back into the reef face and

have sediment down them to the limit of visibility
(Fig. 20). Adjacent rock surfaces have abundant corals, the
same types of species that occur in shallower water. Live
Halimeda is abundant all down the slope to over 60 m.

Hillis-Colinvaux (1980) found four species of Halimeda
on the seaward reef off Mut at 10 to 15 m depth. She
estimated cover of Halimeda on this bottom as about 15%
and commented that Halimeda was much more conspicu-
ous on the spur reef structure than she would have
expected on a reef buttress in Jamaica.

Halimeda flakes dominate the sediments of all leeward
side oceanic reefs. Below about 20 m depth, sediment
builds up on any nearly horizontal surface, particularly near
the reentrants which are the primary "down chutes" for
sediment.

There are many overhangs and small caves formed by
coral plates on the leeward reefs. Incredibly delicate large
colonies of stylasterine corals grow in their dim recesses.
Three species of sclerosponges — Astrosclera willei/ana,
Acanthochaetetes welisi, and one unidentified species
(Basile et al., 1984) — are found in caves along the reef

SUBTIDAL ENVIRONMENTS AND ECOLOGY

115

Fig. 18 Typical views of seaward reefs off the southwestern islands, leeward side of Enewetak. Upper left: Area of heavy graz-
ing, probably by parrot fishes, on rock substrata. Upper right: Seaward end of the reef flat with short "grooves" going Into the reef
flat. Lower left: Heavily greized substrata, with parrot fish tooth marks, depth 2 m. Lower right: Seaward end of the reef flat with
some large coral colonies.

face, but they are small and do not produce significant
amounts of calcium carbonate.

There are many large fan-like gorgonians along the
vertical face, in addition to widely scattered colonies of
antipatharians (black coral). Large black coral "trees" are
rare in these (and all other) areas.

On the leeward side of Enewetak Atoll there is an algal
ridge-type structure which is not well known. Marsh (1970)
reported one area at Igurin to have "a relatively good
growth" of coralline algae. The leeward ridge is in many
places slightly submerged at low tides, but never as emer-
gent as the windward reef flat.

The outer slope of Enewetak below scuba diving
depths was examined to a depth of 365 m with the
research submersible Makali'i during the summer of 1981.
Twenty-two dives were made on the seaward face from
Biken around the southern end of the atoll to south of
Runit (Colin et al., 1986). The seaward reefs of the north-
ern half of the atoll were not examined.

The depth profiles of five areas on the seaward margin
are shown in Fig. 21. The profile of the outer slojie of

Enewetak is very steep, an angle of about 60° between 90
and 360 m with the leeward slope being slightly steeper.
Emery et al. (1954) and subsequent writers have com-
mented on the steep slopes of atolls in the northern
Marshalls. Their opinions were based on echo soundings
and were confirmed by observations from the submersible
Makali'i.

To depths of about 300 m the slojje is generally rock
with small accumulations of sediment. Every near-
horizontal surface has a dusting of sediment, and small
ledges have accumulations varying with the area where
sediment can rest. There is little significant accumulation
of talus to depths of 200 to 300 m from upper areas as
the slope remains steep enough to prevent talus accumula-
tion. Incised, highly polished vertical grooves occur in the
rock face serving for downslope transport of sediment. At
depths between 200 and 300 m, large talus begins to
occur in the form of broken colonies of coral and reef plate
brought down the slope. In some locations a Halimeda
sediment-dominated slope began at about 270 to 300 m
depth with a slight decrease in slope. Along with this were

116

COLIN

Fig. 19 Typical views of seaward reefs off the southwestern islands off Enewetak. Upper left: Rubble area to seaward of the reef
flat, depth 5 m. Upper right: Area of isolated coral heads seaward of rubble area. Lower left: Small acroporid corals on the outer
reef face, southwestern islands, depth 20 m. These corals are probably less than 10 years old and may represent recruits after
major storm damage to the community. Lower right: Outer slope along the southwest islands, depth 20 m. The bottom slopes
away at about a 45° angle to great depths.

often mounds or ridges of talus and carbonate blocks more
than 1 m across.

There was some relief on the rock face at 100 m to
about 180 m, often with the surface pitted with shallow
depressions less than 50 cm across. There were occasional
small caves, seldom penetrating more than 1 m into the
reef face.

Stony corals were observed to grow relatively deep.
Below about 60 m only flattened forms were found. Sparse
coral communities occurred to at least 90 m depth, with
individual colonies occurring to slightly more than 100 m
depth. Similarly, attached and living Halimeda colonies
were found at more than 120 m (HillisColinvaux, 1986).
Green algae were found to almost 150 m and coralline
algae to nearly 200 m.

Some differences in biological zonation were noted
between the windward and leeward slopes. The windward
areas have more coral at 60 to 90 m depths, larger popu-

lations and diversity of small reef fishes from 60 to 200 m,
and generally more benthic invertebrates.

In the wide channel area, there seemed to be much
down slope transport of sediment, although again the steep
slope at 100 to 200 m trapped relatively little sediment on
the face. Below about 200 m, huge slopes of Halimeda
with seapens growing on them were found (Colin et al.,
1986). At the eastern edge of the wide channel this uncon-
solidated slope was alternating elevated areas of talus and
the sand between "pure" sand slopes.

Lagoon Water Column

The waters of the lagoon have not received adequate
attention. Recent work has examined the circulation of the
lagoon (Atkinson et al., 1981), the relationship between
reef-produced organic material and lagoon plankton
(reviewed in Gerber and Marshall, 1982) and plankton

SUBTIDAL ENVIRONMENTS AND ECOLOGY

117

Fig. 20 Reentrants of the reef face, southwest islands, Enewetak Atoll. Upper left: Sand channel in between elevated reef fingers,
depth on sand approximately 12 m. Upper right: upper portion of a reentrant on the outer slope at 30 m depth. Lower left and
right: Sediment transport via reentrant down the outer reef face, southwestern islands, depth 40 m.

composition (Gerber, 1981), but these are only a bare
beginning. The open lagoon is generally rough and less
than ideal for working in a small boat. Navigation and posi-
tioning are difficult because the islands of the atoll rim are
so low that in the center of the lagoon little land is visible.

Water column productivity within the lagoon has not
been well documented. The author has seen, on several
occasions, large blooms of phytoplankton in the northern
and western lagoon. These were sharply differentiated
areas of "brown water" many kilometers in length and, on
two occasions, as surface slicks many centimeters thick.
The surface slicks occurred under extremely calm condi-
tions and were nearly linear masses of tan phytoplankton,
tens of meters broad and over 1 km in length. Thickness
was not determined but was believed to be at least 30 cm.
The blooms may be associated with water of lengthy
residence time in the lagoon since they have been
observed only from areas where this is typically the case.

Dense swarms of zooplankton were often observed in
the lagoon by scuba divers, often within a discrete portion
of the water column. During the summer, particularly huge

numbers of salps and ctenophores were observed many
times on the reef. Gerber and Marshall (1982) documented
a "bloom" of pteropods and a subsequent population
decrease in the central lagoon during a 4-week period.

The unidirectional flow of water from windward reefs
across the reef flat to the lagoon is significant not only in
the physical flushing of the lagoon but as a mechanism by
which increased nitrogen, produced by nitrogen fixation on
the shallow reef flat (Webb et al., 1975), reaches the
lagoon.

Webb et al. (1975) felt there were three important
routes by which Calothrix Crustacea fixed N2 enters the
remaining reef ecosystem. First, fish grazing and the low
assimilation efficiency (Chartoch, 1972) of herbivorous
fishes makes the fixed nitrogen available. Second, fragmen-
tation of Calothrix and lagoon transport makes it available
to herbivores and detritivores in the lagoon. Third, 40 to
60% of the nitrogen fixed is released in solution and is
available for other organisms.

Gerber and Marshall (1974) have shown that detritus
flowing off the shallow reefs forms a major component of

118

COLIN

Fig. 21 Views of lagoon reef coral colonies. Upper left: Turbinaria sp. Upper right:
Large colony of Pontes rus with areas removed, possibly by coral damaging fishes.
Lower left: Pontes nts, large colony showing a shift from columnar to plate-like
growth with depth and exposure to less light. Lower right: Fan-like growth form of
the hydrozoan Millepora sp. on a lagoon margin.

ingested material in two abundant lagoon zooplankters.
Furthermore, lagoon copepods have also been known to
ingest and assimilate such particulate matter (Gerber and
Gerber, 1979).

Gerber and Marshall (1982) suggest that the
occurrence of planktonic organisms in the central lagoon
results mostly from production and consumption in the
water-column community. They indicate that the reef com-

munities are the sources for a large percentage of the car-
bon and nitrogen fixed and present in lagoon waters. The
phytopiankton community of the lagoon is also important
as a food chain base, but the relative importance of each
is not well understood.

Gerber (1981) documented the diversity and abun-
dance of zooplankton at two stations in the lagoon. Ninety
six species of copepods and species of chaetognaths, larva-

SUBTIDAL ENVIRONMENTS AND ECOLOGY

119

ceans, mysids, euphausiids, amphipods, siphonophores,
pteropods, dinoflagellates, medusae, other planktonic Crus-
tacea and larval forms were found. One station, near the
Enewetak-Medren reef flat had lower abundance, fewer
species of typically planktonic organisms, and more mero-
planktonic and benthic forms than the mid-lagoon station.
There was considerable variation in densities of zoo-
plankters among samples taken during the same study
periods of a few to several weeks at the mid-lagoon sta-
tion.

Gerber and Marshall (1982) reported that lagoon con-
centrations of copepods, pteropods, and larvaceans were
higher during their summer sampling period. Phytoplank-
ton biomass in mid-lagoon in summer was also about twice
that of the winter. Individual components of the zooplank-
ton changed their densities considerably during periods of
several weeks during the summer. Copepods and larva-
ceans increased 1.5 to 3 times. Pteropods increased 20
times in 4 weeks, then declined rapidly.

Coles and Strathmann (1973) collected mucus floes
from the water column at Enewetak and other areas and
found them to represent substantial quantities of organic
matter when compared to particulate organic material in
the water. They noted that under calm conditions few
mucus floes were seen in the water at Enewetak, but after
a storm abundant large floes were seen passing into the
lagoon.

ASPECTS OF MARINE COMMUNITIES
AT ENEWETAK

Reef Growth and Destruction

Coral reef growth is a balance of factors: the accretion
of calcium carbonate by stony corals and other calcifying
organisms in addition to the consolidation of these materi-
als into a cohesive structure versus the erosive effects of
grazing pressure, physical weakening, and destruction of
the reef structure. Much work at Enewetak has focused
on questions related to the growth and maintenance of
reefs. Not all is summarized here but some environmental
factors concerning reef growth are.

Calcification of corals and other organisms can be
affected by environmental conditions, such as light, tem-
perature, and water movement over the range of condi-
tions under which the organism ca.i survive. Stony corals
are also known to "compete," albeit in a relatively slow
manner. Methods include overgrowth, reducing the light
necessary for calcification and growth of competitors, and
by "extracoelentric digestion" in which mesentarial fila-
ments are extended to "attack" and kill tissue of other
species growing close by.

The range of conditions inhabi*3d by a single species
or genera of corals is often broad. The genus Pocillopora is
illustrative. Stimson (1978) reports that the Pocillopora
species at Enewetak occur over a broad range of depths
but are most abundant on reef flats and in water <5 m
deep with currents. He reported P. verrucosa to reach

15 m depth on pinnacles and windward and leeward reef
slopes and to occur in the "small head zone" (Odum and
Odum, 1955) north of Japtan.

Pocillopora uerrucosa is also common in spur and
groove areas of the windward reefs. In eastern Australia
the species is found in areas of regular water movement
and good illumination, and its growth variations are less
diverse than those of P. damicornis (Veron and Pichon,
1976). Pocillopora damicornis occupies potentially a
greater range of habitats than any other coral at Enewetak.
Veron and Pichon (1976) have figured the wide variation
in corallum morphology and documented the broad range
of conditions this species inhabits in eastern Australia.
Pocillopora eudoxt^i occurs deeper than any other branch-
ing coral at Enewetak, to approximately 60 m on the sea-
ward slope.

Members of Acropora are similar. Some are limited to
very shallow water. Stimson (1978) found A. aspera and
A. humilis only in water less than 2 m deep. Acropora digi-
tifera and A. aspera are sometimes exposed and killed by
extreme low tides. Others, such as A. s^ringoides, are re-
stricted to water deeper than 5 m. Acropora s\;ringoides is
abundant on the flanks of patch reefs and pinnacles near
Enewetak and Medren. Other species have broad depth
distributions. Acropora hyacinthus and A. nasuta occur
from 1 to 20 m depth.

Coral growth rates have been examined for a number
of species of stony corals at Enewetak. The technique of
x radiography of slabbed coral specimens was first applied
to Enewetak coral specimens and used to verify the annual
nature of the density banding observed (Knutson et al.,
1972). Autoradiographic exposures of coral slabs show dis-
tinct bands of activity from atomic test series and, there-
fore, serve as bench marks in coral growth chronology.
Knutson et al. (1972) also presented evidence that the
high density bands seen were formed during the rainy
season at Enewetak. Buddemeier et al. (1974) examined
skeletal growth rates of 15 species of corals, including the
same species from various locations at Enewetak. They
reported growth rates of generally 4 to 12 mm per year
with some exceptions above and below these figures. Not
all coral species examined showed variation in growth
rates with depth. Porites lutea did show a negative correla-
tion between growth rate and increasing depth, with about
one-half the rate at 25 to 30 m as was at 4 to 10 m
depth. However, colonies of Goniastrea sp. collected in
deep water grew as fast as those from shallow water.

Although Buddemeier et al. (1974) focused attention
on obtaining large symmetric head corals, no specimens
examined indicated ages before 1952 and 1953. Whether
the nuclear tests of 1952, particularly the "Mike" test, had
any effect on this is uncertain.

Stimson and Polacheck (MPRL, 1977) reported that
growth rates of Acropora and Pocillopora at four different
depths from 1 to 15 m on lagoon pinnacles and patch
reefs were statistically indistinguishable. Three species of
common shallow water Acropora had annual increments in

120

COLIN

diameter of the colony of about 5 to 6 cm, whereas
Pocillopora in shallow water had an annual growth in diam-
eter of about 4 cm. Smith and Harrison (1977) reported
table Acropora colonies to increase their diameter 15 cm
or more per year once they had reached the stage where
they transform from a vasiform to tabulate corallum.

Haggerty (1980) found that with increasing water
depth both Fauia pallida and F. stelligera had more widely
spaced corallites, a slower linear skeletal growth rate and a
decrease in the annual skeletal growth rate per square cen-
timeter. Fauia pallida had a hemispherical colony form in
all environments at 3 to 41 m depth, but deep water
populations possessed more septa per corallite than shal-
low water. Fauia stelligera changed its colony morphology
with depth, from "lobate or hummocky" in shallow water
to "columnar with a slight basal skirt" (Haggerty, 1980) in
deeper water.

Stimson (MPRL, 1973) looked at the interactions via
"extracoelentric digestion" between closely adjacent corals
of various species at Enewetak. In the hierarchy of
Enewetak corals, based on the species which could be
successfully "attacked," Astreopora mi/riopthalrna ranked
the highest, with Acropora acuminata second, and Pontes
lutea third. Pocillopora spp. were lowest, being killed on
contact with other species.

Stimson (1978) studied the timing of planulation by
species of Pocillopora and Acropora at Enewetak. He
found Enewetak colonies to produce planulae primarily
during the new and first quarters of the moon. He also
suggested that planulation by Acropora may be more sea-
sonal than Pocillopora because about twice as many
colonies planulated during the summer than in the winter.
Among pocilloporids, colonies 6 to 8 cm in diameter (15
to 30 cm in volume) were the smallest observed to planu-
late and estimated to be 1 to 2 years old. Acropora
colonies as small as 50 cm'' planulated, but most were
greater than 1000 cm^ in volume. Pocilloporids generally
produced more planulae than acroporids at Enewetak.
There can also be geographic variation in lunar timing of
planulation. The lunar periodicity of planulation in P. dam-
icornis is the same in Palau as Enewetak but is reversed
from Hawaii (Stimson, 1978).

Stimson (1978) felt that shallow-water corals at
Enewetak were in a more "disturbed" environment than in
deeper water and that species found predominantly there
would have high reproductive rates. He has measured
annual mortality rates as high as 20% for shallow-water
corals. Most of these species produce planulae rather than
smaller eggs and may do so to facilitate rapid settlement in
the current-swept reef flat areas.

The large table Acropora (A. hiiacinthus?) produce
shaded area beneath them. Stimson and Polacheck (MPRL,
1977, 1979) found the shaded area to be less than 1 m^
per colony at 30 to 80 cm from the substrate. The density
and number of other coral species beneath table Acropora.
both dead and alive, were less than in controlled unshaded
areas. The genera of corals occurring in the shaded areas
v^ere Stiilocoer)ieUa, Montipora, Seriatopora. and various

massive species Species of Acropora and Pocillopora
piedominated the adjacent unshaded areas.

Kastendick (MPRL, 1975, 1976) examined the habitat
differences among eight species of fungiid corals which
grow unattached on lagoon coral pinnacles and patch reefs.
The young of two species were attached (Fur\gia fungites
and Halomitra pileus) and found almost exclusively at the
upper limit of adult distribution. It is likely that as they
age, fungiids move passively down the slope. Kastendick
observed invasions of colonies onto the foot area of several
pinnacles after removal of these corals the previous year.
Fur^gia spp. were found exclusively on coral rubble,
whereas H pileus was most abundant on sandy substrate.
Translocation of individuals up and down the pinnacle
slope indicates that F. fungites has the most restricted
habitat requirements, with H. pileus less so.

Storms during the summer of 1972 (Nolan, 1975;
Stimson, MPRL, 1974, 1976) destroyed large areas of
coral growth on reefs with a southern exposure, even
within the lagoon. Only massive species of Porites survived
in any quantity on damaged reefs. First recolonizers were
Acropora striata and A. s^ringoides. Stimson (MPRL,
1975, 1976) also noted that Sarcoph^ton sp., a soft coral,
was an important colonist and component of the benthic
fauna on storm-damaged reefs. As the hard coral commu-
nity recovered, he believed that Sarcoph\^ton sp. would
become progressively rarer. It was observed shading many
corals, including P, damicornis and Seriatopora hystrix.

Highsmith (1981a) suggested that corals with high
skeletal density are less able to recolonize dead areas on
their skeletons by tissue growth than less dense species.
For example, he reports Porites lutea. with a relatively
low density (1.4 to 1.5 g cm~^), is able to rapidly grow
over dead skeletal regions, whereas Goniastrea retiformis
(1.6 to 2.0 g cm~^) requires considerable skeletal deposi-
tion and polyp growth reorientation to overgrow dead
areas.

Calcareous material produced by organisms other than
stony corals is important in both the reef framework and
sedimentary material. Animals, other then Scleractinia,
which might make a significant contribution are the Fora-
minifera, Mollusca, Bryozoa, Sclerosponges, and other Cin-
daria.

The occurrence of foraminifera tests in sedimentary
material in the lagoon and beach sands at Enewetak is well
documented (Emery et al., 1954; Odum and Odum, 1955;
Deutsch and Lipps, 1976). Forams may consititute a sig-
nificant percentage of lagoon sediment grains, but they are
believed insignificant in reef growrth. Mollusc shells similarly
constitute a minor component of lagoon sediments but do
not contribute to reef growth.

Cuffey (1973) found no bryozoans on the coralline
algal ridge of Enewetak and very few in the area (which he
terms the "back-ridge trough") immediately shoreward of
it. The reef flat, similarly, has almost no bryozoa occurring
on it. Areas between islands with abundant coral in shallow
water also had relatively few bryozoa. Howpver, in the
lagoon margin area, where larger patch reefs begin to

SUBTIDAL ENVIRONMENTS AND ECOLOGY

121

occur, bryozoans increase in abundance, particularly in the
patch reefs at depths below 9 m. Cuffey (1973) believed
the floor of the deep lagoon, accessible to him only by
dredging, lacked any diverse bryozoan fauna and only a
few "small detrital fragments" of bryozoa were taken. The
pinnacle reefs of the deep lagoon, however, contained an
abundant and diverse complement of bryozoans. The steep
leeward slope of the atoll apparently had the most diverse
community of bryozoa, particularly below 9 m.

Cuffey (1973) found bryozoans more abundant in Ber-
muda than Enewetak, where they were infrequent in
depths less than 9 m. He suggested that the considerably
higher diversity of Enewetak corals might adversely influ-
ence the relative success of bryozoa when compared to
Bermuda. He makes the interesting observation that "the
leeward (southwestern) side of Eniwetok Atoll harbors
noticeably more bryozoans (both taxa and individuals) than
does its windward (northeastern) side. Bryozoan distribu-
tion on Eniwetok thus parallels sponge distribution within
Pacific atolls, as described by De Laubenfels (1954)." In
addition to not being principal frame builders on Enewetak
reefs, bryozoans do not contribute any significant amounts
of classic detritus to the sediments of the reef (Cuffey,
1973).

Cuffey (1973), in considering the bryozoa of Enewetak,
found that most species inhabited the undersides of corals
and rocks on reefs. The most abundant bryozoans at
Enewetak were encrusting cheilostomes which grow as
thin, sheet-like crusts on the undersurfaces of corals or
rocks. Most bryozoa inhabited these sheltered microhabi-
tats and "function primarily as 'hidden encrusters,' adding
small quantities of calcareous skeletal material to the reef
framework." Cavity-filling tendency by bryozoa was noted
in Bermuda but not at Enewetak.

Hydrozoans of the genus Millepora are extremely
important calcifying and reef-building organisms at
Enewetak. In many areas, such as the large coral head
zone of Odum and Odum (1955), Millepora spp. can form
heads several meters across which grow to the low tide
level where they form flat-topped structures (Fig. 12). In
deeper water — including the ocean slopes of all sides,
lagoon margin patch reefs, and lagoon pinnacles —
Millepora spp. form large delicately branched, often fan-
like, colonies (Fig 21).

The stylastcrine hydrozoans, unlike Millepora, are not
important carbonate producers at Enewetak. The delicate
fan-like species of Sfy/aster are found beneath overhangs
and within caves of patch reefs, pinnacles, and on outside
reefs. Similar, but more robust, are two species of
Distichopora which occur in similar areas but are often
more exposed. These are extremely common on the
leeward reef slope but do not produce reef framework.

Tubipora musica is not common at Enewetak, being
found only occasionally on reef fronts or on lagoon pinna-
cles, and therefore does not produce significant reef struc-
ture.

Calcareous green algae, particularly members of
Halimeda, are extremely important in sedimentation and

reef building. The distribution of Halimeda in most subtidal
environments at Enewetak is well documented (Hillis-
Colinvaux, 1977, 1980, 1986; Emery et al., 1954; Colin,
1986). Borings at various atolls (Funafuti, Enewetak,
Bikini, reviewed by HillisColinvaux, 1980) have shown
Halimeda segments to be the major identifiable component
of unconsolidated lagoon deposits. Milliman (1974) indi-
cates that among sand-sized components of lagoon sedi-
ments in Pacific and Atlantic atolls, Halimeda segments are
generally the first or second most common material. Hillis-
Colinvaux (1980) cites evidence in Couch et al. (1975)
that Halimeda segments make a significant contribution not
only to unconsolidated lagoon sediments but also to
material underlying the reef rim. The fate of Halimeda
plates in sediments varies. Some are shed intact, but a few
species (H. macroph^sa and H, favulosa at Enewetak) have
delicate segments that are easily broken (HillisColinvaux,
1980).

Carbonate production rates by Halimeda at Enewetak
are not well known, depending on plant density, generation
time, and shedding rates. HillisColinvaux (1980) reports
that population densities in Halimeda can vary by two or-
ders of magnitude with concurrent effects on carbonate
production. Turnover rates are perhaps lower than some
published data (HillisColinvaux, 1980) since Halimeda is
predominantly a long-lived alga. One experiment at
Enewetak indicated that 70% of the original thalli were still
present after 4 months (HillisColinvaux, 1980).

Dense populations of Halimeda at Enewetak and else-
where have about 100 plants m^^ of the H. incra^^sata-
c\;lindrica type thallus. The rock-growing H. opuntia type
can have higher densities of plant material, although abso-
lute numbers of plants may be less. HillisColinvaux (1980)
estimates that the H. incrassatact^Hndrica types would pro-
duce only about 10% of the total carbonate accumulation
in the lagoon (Smith and Kinsey, 1976) if they covered the
major portion of the lagoon bottom. She was not aware at
that time of the presence of the "Halimeda meadows" and
the estimated percent coverage of the deep lagoon bottom
predominantly by Halimeda. The contribution of Halimeda
segments from lagoon pinnacles may be smaller than
HillisColinvaux (1980) calculated when a comparison was
made to Halimeda from flat lagoon bottoms.

Bioerosion of Reefs

The agents of bioerosion at Enewetak act in a variety
of ways. Some, such as the boring sponges of the genus
Cliona, excavate chambers on the carbonate skeletons of
living corals and virtually any other carbonate substrate.
The shells of molluscs, coral rubble, and other small car-
bonate fragments can be attacked. Other organisms, in the
course of feeding activities, rasp away the surface layers of
carbonate while grazing the thin film of algae which covers
such surfaces. The parrot fishes, surgeonfishcs, various
echinoderms, and other such herbivores generally pass the
carbonate material through their gut, subjecting it not only
to mechanical effects but also to chemical effects. Other

122

COLIN

organisms may prey directly on calcifying organisms and in
the proces% often damage the skeleton.

A few species of fishes vigorously attack coral skele-
tons, biting off and ingesting the tips of branched species.
Randall (1974) observed the pufferfish, Arothoron nigro-
punctatus, feeding heavily (85 to 100% of gut contents) on
corals, particularly Acropora and Montipora. Hiatt and
Strasburg (1960) found corals in the guts of nine plectog-
nath fishes (two triggerfishes, three filefishes, three puffers,
and one sharpnose puffer). Most, but not all individuals of
any species, had ingested branched coral tips in various
amounts. Although none of these fishes are obligate coral
predators, many contain coral tips in such quantity that
these must constitute a regular part of their diet at
Enewetak (Randall, 1974).

Large portions of coral skeleton will, on occasion, have
the ends of the branches removed, often with piles of coral
fragments left in the depression. This is seen in Pontes rus
at Enewetak (Fig. 21) and in other species. It is assumed
this phenomenon results from the activities of fishes which
feed on coral branches, but the feeding by some of these
fishes is seldom observed.

Other coral-feeding fishes tend to eat only the polyps,
leaving the skeleton essentially intact. In such cases, the
polyp normally regenerates. A number of butterflyfishes
(Chaetodontidae) and damselfishes of the genus
Plectrogli^phidodon feed on corals in this matter (Motta,
1980; Randall, 1974; Reese, 1973, 1975, 1977) and are
discussed elsewhere. Randall (1974) notes also that the
blenny Ecsenius bicolor at Enewetak has been observed
feeding on Acropora.

Some herbivorous fishes occasionally scrape at the sur-
face of living corals doing more damage than the chaeto-
dontids. Scarids produced a characteristic scrape mark on
corals with an elongate furrow, often with a slight ridge
along its midline where the two sides of the beak fuse.
Hiatt and Strasburg (1960) found some species of scarids
at Enewetak had fed on corals. Randall (1974) has
reviewed the question of parrot fish grazing on live corals
and discusses an apparent disparity between published
data on coral feeding by scarids at Heron Island, Great
Barrier Reef, and Hiatt and Strasburg's (1960) information.
He found no obvious reason for the differences observed
but suggested that local coral cover may influence how
much coral is ingested by parrot fishes. Although some
scarids do graze live corals, the impact of this behavior is
probably minor compared to the effect on sediment pro-
duction and deposition.

Randall (1974), Ogden (1977), and others have docu-
mented the role of scarids in sediment production. The
rasping of rock or coral for its algal film is the first step.
This material is then ground to a fine consistency by the
pharyngeal mill of the parrot fish, passed through the gut,
and eventually expelled. The rain of sedimentary material
shed when parrot fishes defecate is impressive, and the
amount of sediment produced from hard substrates by this
mechanism is enormous.

Also important in sediment production are fishes which
reduce the hard parts of invertebrates (mollusc shells, echi-
noid tests and spines, crustaceans, etc.) to bits. Randall
(1974) reports that plectognaths with their fused or but-
tressed teeth, lethrinids with molariform teeth, labrids with
pharyngeal teeth, and dasyatid and myliobatid rays with
plate-like jaws are well adapted for this purpose.

Massive corals at Enewetak are attacked by a number
of biological agents. Although seldom visible, these agents
weaken the skeleton to the point that physical factors can
break the colony loose or cause it to crumble. Highsmith
(1981a) reports that clinoid sponges accounted for 70 to
80% of skeletal damage in various massive corals from
Enewetak. They did not burrow deeply into the skeleton,
only a few millimeters, but extended interconnected
chambers laterally beneath dead surfaces of the coral
colony. Highsmith (1981a) reported that 65 to 95% of the
boring was within the "dead area" of skeletons. In a mas-
sive coral this "dead area" includes the area around the
basal attachment and dead spots on the colony surface.
Similarly, these dead areas are heavily eroded by grazing
organisms. When exposed to light or scraped (as when
overlying skeletal material is removed), clinoid sponges
engage in rapid burrowing (Ruetzler, 1975) Heavy grazing
pressure, combined with this response, may produce rapid
erosion rates at basal attachments.

Highsmith (1981a) points out that skeletal weakening
at the base, combined with storm-induced water motion,
may not be sufficient to dislodge most massive colonies.
However, coral rubble on the bottom can be put into
motion by storm waves and, to a point, may be the most
significant force in breaking heads loose. Eventually
though, "as massive corals grow, they become more sus-
ceptible to breakage by storm currents and less susceptible
to breakage by suspended rubble or to biocrosion detach-
ment."

The alpheid shrimps occurring in deep grooves on
Goniastrea retiformis apparently form the grooves, not by
boring or erosion, but by preventing growth of coral in
that area while the remainder of the colony continues to
increase in size (Fig. 22). These grooves, though, provide
dead areas which penetrate deeply into the G. retiforrr}is
head and are penetrated by boring organisms (Highsmith,
1981a).

Highsmith (1981b) suggested that bioerosional damage
to corals is positively correlated with increasing skeletal
density. Five species of Enewetak corals {Ouloph\^lha
crispa. Fauia pallida, Goniastrea retiformis. Pavona clauus,
and Pontes lutea) had a positive correlation between
bioerosion and density. This correlation did not correspond
to differences in growth rates. The slowest growing
species, F. pallida, was the least bored.

Among molluscs, the boring bivalve Lithophaga curta
preferentially colonized the coral Montipora berrvi
(Highsmith, 1980). Boring bivalves in general have thin,
weak shells and, if exposed, are easily eaten by fish preda-
tors. Highsmith (1981a), for example, reported that the

SUBTIDAL ENVIRONMENTS AND ECOLOGY

123

Fig. 22 Heads of Goniastrea retiformis with deep grooves formed through the
activities of alpheid shrimps.

wrasse Thalassoma lutescens readily took bivalves exposed
during collecting. Bivalves were not common borers of
Enewetak massive corals. Highsmith (1981a) found only
four bivalves in more than 100 coral heads and contrasts
with other areas where they produce significant boring.

Polychaetcs are also significant borers of corals
(Highsmith, 1981a) but are often believed to occupy
empty sponge chambers. He found 280 polychaetes in a
single Pontes lutea head; the diversity of polychaetes
exceeded any other infaunal organisms. Although they arc
common, they are probably not as important borers as are
sponges. Sipunculans were imp>ortant borers of coral rub-
ble, rather than live coral (Highsmith, 1981a).

Highsmith (1981b) discussed the role of endolithic
algae, Ostreobium spp., in several species of Enewetak

corals. They occur as one or more dark green bands in the
upper few centimeters of the coral skeleton. He found
Ostreobium in every coral sampled from the surface to
30 m depth. No significant effect by the filamentous algae
on the integrity of coral skeletons was detected. In some
species of corals there was an inverse correlation, with
considerable variation, between water depth of a coral and
the depth of its outermost algal band. Algal bands are
believed to occur where and when conditions are suitable
for vigorous growth.

DiSalvo (1969) isolated bacteria from within the skele-
ton of the coral Pontes lobata. Bacteria were cultured from
light brown discolored regions revealed when the corals
were split of>en. Attempts to culture bacteria from adja-
cent, nondiscolored skeleton were not successful. Some of

124

COLIN

the isolated bacteria were able to digest chitin in vitro, and
DiSalvo suggested these might weaken the skeleton by
breaking down the organic matrix. DiSalvo (1969) also
found that sediments in proximity to the bases of corals
had 10^ to 10* bacteria g dry wt^' of which 10 to 20%
were chitin-digesting varieties. Thus there is a ready
source of suitable bacteria close to the coral's skeleton.

Invertebrate Coral Predators

Some invertebrates are also coral predators at
Encwetak. The crownofthorns starfish, Acanthaster plana,
is found in many areas. Most lagoon pinnacles have one or
more A planci, and evidence of their feeding activities on
corals is apparent. The status of A. planci populations at
Enewetak, population changes, and impact on the reefs in
recent times arc not well known. Population levels are cer-
tainly below "plague" levels. Allen (1972b) noted that
"during the summer of 1970 the author observed an
increasing number of Acanthaster at Eniwetok. Prior to this
date relatively few were observed." Starck (MPRL, 1971,
1972) found "no unusual populations of Acanthaster."
About a Ihour (one dive) search during the day would
result in one to two individuals, whereas a nighttime
search of about only one-quarter the area revealed 10 to
12 specimens. Starck found a wrasse, Cheilinus undulatus,
of about 50 kg weight (of four examined) with a large,
nearly intact A. planci in its gut contents. Starck, in Ander-
son (1979), reported a sizeable population, perhaps as
many as 50 to 100 A planci on Pole Pinnacle, but stated
that there was no extensive damage to the coral there.

The juveniles of A planci apparently occur beneath
rubble on reefs. Lisa Boucher and Scott Johnson (personal
communication) report finding numerous examples from a
few centimeters to less than 1 cm disk diameter on pinna-
cles near Enewetak Island. Although they never found
these A. planci to be very common, a distinct increase in
the numbers of juveniles encountered was noted in April
1982.

Storm Destruction of Reefs

The effect of subtyphoon storms (tropical storms, tropi-
cal depressions) on subtidal environments can be devastat-
ing. Many such storms occur compared to full strength
typhoons and are often not noted in historical records.
Damage from wind and rain to terrestrial areas may be
minor, but the swell produced by such storms can wreak
havoc in shallow-water communities. The production of
boulder ramparts by storms is well known, and such struc-
tures occur on the southwest islands of Enewetak. Since
the ocean shores of these islands are normally in the lee
and the reef slope is steep and close to shore, the infre-
quent reversal of wind and waves can cause catastrophic
destruction of corals in shallow water, moving vast quanti-
ties of material onto the shore or into deeper water.

The movement and effect of ocean swells in the lagoon
are important. The wide pass at Enewetak is sufficiently
deep to allow ocean swell from the southeast to southwest

to enter the lagoon. Ocean swell is also refracted at the
pass so that wave trains moving from the west and
southwest can come through the pass and proceed north
to northeast to reach lagoon shores. These long period
swells have no direct effect on the deep lagoon communi-
ties. However, when they reach the lagoon shore of wind-
ward islands or shallow pinnacle or patch reefs, they can
turn these shallow-water communities into churning mael-
stroms of breaking waves. One such period of swells from
the southwest to west for 3 days in July 1982 turned the
lagoon shore of Enewetak Island and other southern
islands into a mass of dark brown water (with essentially
zero visibility) above 6 m depth with breakers to 2 m high
where depths were less than about 4 m. Significant swells
and breakers persisted for nearly 1 week. Many fishes,
molluscs, and other invertebrates were killed and cast up
on the beaches. In places sediment and rubble were eroded
away as much as 1 m or more. Many of the delicate corals
on shallow reefs {Pocillopora edouiixi. Millepora spp.) were
broken to stubs.

Carbonate material from shallow water, particularly
large pieces such as coral boulders, can be deposited on
the islands by storm waves, transported into the lagoon or
transported downslope on the seaward reefs. Various
islands of Enewetak are densely covered with recently
deposited coral boulders, and boulder ramparts are evident
on the seaward beaches of some islands. Less visible, but
perhaps more significant, is downslope transport of rubble
on seaward reefs. Talus was evident at many locations
examined by the submersible Makali'i. and shallow-water
coral rubble was extremely evident in the material photo-
graphed. At 300 to 360 m, the maximum depths visited,
the slope of the bottom was generally too steep for
extremely large talus accumulations. Larger accumulations
of talus should lie below those depths where the bottom
slope is less steep.

Storm swell within the lagoon may be a major factor
controlling the morphology of lagoon margin patch reefs
and shallow pinnacle reefs. Choptop Reef had moderate
damage from swells entering the lagoon in July 1982.
Some large coral heads, their bases weakened, were tum-
bled over. Pieces of Porites c^ilindrica colonies as much as
a meter across were torn loose from larger colonies and
rolled a few meters over the bottom. Although individual
branches were often broken, such pieces formed satellite
patches of P. cfjiindrica which survived and grew. In
another instance, a tunnel torn through a huge mass of
P. ci;lindhca at Choptop Reef during a tropical storm in
March 1981 was collapsed by the July 1982 storm. In
both instances the total structure was fractured. Swell
within Enewetak Lagoon seems capable of breaking apart
patch reef features which reach too near the surface.
Where the internal structure of lagoon margin patch reefs
is visible, they seem little more than accumulations of
poorly cemented coral rubble. One well-known lagoon pin-
nacle, "Tunnel Pinnacle" (Fig. 6), has had the "tunnel" col-
lapsed, almost certainly by storm swell, during the past
few years. Reese (1981) provides a description of the

SUBTIDAL ENVIRONMENTS AND ECOLOGY

125

effects of storms on the corals and butterflyfishes (chaeto-
lontidae) of "Tunnel Pinnacle" and other pinnacles at
Enewetak.

The lack of significant cementation on upper surfaces
of patch reefs and pinnacles by coralline algae may reduce
greatly the amount of wave energy required to crumble
the structure. Protection of the lagoon from storm swells
by a complete or nearly complete atoll rim with no deep
passages may also be important. At Ujilang Atoll, which
has no major passes allowing ocean swell to enter the
southern lagoon, lagoon margin patch reefs examined were
near planar on top at about the level of spring low tides
with near vertical edges dropping to a few meters in
depth. The patch reefs were well cemented by coralline
algae on their upper surfaces. Ujilang is exposed to essen-
tially the same oceanic conditions (wind, waves, and
storms) as Enewetak, yet patch reefs of such morphology
are not found at Enewetak.

Herbivory in Subtidal Communities

Much work on herbivory and its impact on the ecosys-
tems at Enewetak has been undertaken on intertidal areas
because of the large, accessible area of such environments
at the atoll and the abundance of herbivores there. Her-
bivory is a major factor influencing subtidal communities in
the lagoon and on the seaward margin. Evidence of intense
grazing pressure can be found in many subtidal areas, both
on hard and soft substrata.

Unlike the intertidal areas, subtidal areas are accessible
to herbivores at all times. On exposed rock substrates,
both seaward and lagoonward of the reef flat at Enewetak,
tooth marks from the action of grazing fishes arc nearly
ubiquitous in areas to at least 15 to 20 m depth. Many
show considerable erosion from grazing (Fig. 23) with
angular facets on the rock, and deep tooth marks, particu-
larly from large parrot fishes, are often densely grouped.

The principal grazers of hard substrates at Enewetak
are fishes, particularly parrot fishes (Scaridae) and sur-
geonfishes (Acanthuridae). In general, algae on subtidal
rock surfaces are close-cropped except in the case where
the species may be heavily calcified {Halimeda spp.) or
potentially toxic or distasteful {Li;ngbia sp). In this respect
subtidal rock surfaces are not qualitatively different from
the intertidal reef flat. Macroalgae and algal films are found
at all depths in the lagoon.

Surgeonfishes show a well-defined zonation among this
largely herbivorous family. Acanthurus triostequs. A. achil-
les. A guttatus. and A. lineatus are principally found on
windward and leeward reef flats, back reef areas, and the
spur and groove zone — all shallow-water environments. In
somewhat deeper water on the seaward reefs, lagoon
patch reefs, and pinnacles are found species of
Ctenochaetus, Zebrasoma, Acanthurus nigrofuscus. and
A. olivaceus. Two species of Acanthurus at Enewetak,
A. thompsoni and A. bleekeri, feed on zooplankton as do
most species of Naso. The species of surgeonfishes with a
well-developed gizzard-like stomach commonly feed on

sediment bottoms and ingest, along with the algae, consid-
erable sand (Randall, 1956; Hiatt and Strasburg, 1960)
(Fig. 23).

Herbivory occurs on sediment bottoms where macroal
gae and microalgac occur. Macroalgae can occur as dense
stands, as exemplified by the species of Halimeda and
Caulerpa, and many are probably unpalatable to her-
bivores. Microalgae can occur as nearly invisible films on
sediment grains on the surface of the sediment bottom, but
easily apparent films (algal mats) covering many square
meters are often found from a few meters depth to the
deepest portion of the lagoon. Epiphytic algae also grow
upon larger algae and are often more desirable to her-
bivores than the plants on which they grow.

Invertebrate grazers of rock surfaces are not as impor-
tant at Enewetak as in the western Atlantic. In many
Caribbean locations sea urchins, particularly Diadema antil-
larum, are extremely abundant and as herbivores have an
impact equaling, or exceeding, that of fishes (Ogden,
1976; Ogden and Lobcl, 1978). Sea urchins, particularly
diademnids, are not nearly as abundant, except in localized
areas, on reefs at Enewetak.

Fishes are important herbivores of sediment bottoms at
Enewetak. The principal herbivore families of hard
substrata, parrot fishes and surgeonfishes, range onto sedi-
ment bottoms also (Fig. 23). Different species from those
that remain on the reef are often involved. As distance
increases from the shelter of the reef, the grazing pressure
of reef-based herbivores decreases. They are exposed to
increasing risk of predation with increasing distance from
shelter. Therefore, soft bottoms near reef shelter are more
heavily grazed, often to the extent that no visible bcnthic
plant growth except the less desirable species mentioned
previously occur near the reef. This results in a
phenomenon most easily visible from the air in which reefs
arc surrounded by a light colored band, compared to sedi-
ment substrata farther away, representing the denuded
substatum close to the reef. This area has been termed a
"halo" (Randall, 1965) or the "Randall zone" (Ogden,
1976) and is a feature found near both Indo-West Pacific
and Atlantic reefs.

Other herbivores, particularly invertebrates, exist far
from the reef, either remaining on or above the sediment
surface or burying and burrowing in the bottom. Dense
stands of macroalgae provide excellent shelter for small
herbivores, both fishes and invertebrates which can hide
among the thalli. Although these macroalgae are not pri-
mary foods for these herbivores, the environment created
provides abundant spaces for epiphytic algae (and cpizoic
organisms also) w)iich are suitable for the small herbivores.

In areas without dense algal cover, burrowing organ-
isms can function as herbivores without the need of
shelter. Irregular sea urchins (Spatangoidea) occur abun-
dantly in Enewetak Lagoon sediment bottoms and
apparently process sediment grains for the algal matter on
their surfaces. These and other "sediment processors"
must pass relatively large amounts of sediment to obtain
sufficient organic matter.

126

COLIN

1>-

Fig. 23 Upper left: Heavily grazed rock sub-
strata, seaward reefs. Right: Heavily grazed dead
coral skeleton on seaward reefs. Lower left: Sur-
geonfishes, probably Acanthurus mata, feeding on
algal films on sediment substrata near lagoon
margin patch reefs.

Other herbivores are found on the sediment surface.
The gastropod, Strombus luhuanus, can occasionally occur
in localized high densities over open sediment in water
2 to 10 m deep. Densities more than 10 individuals per
m^ with distinct (advancing?) edges to the population were
often seen with adjacent areas lacking S. luhuanus. High
densities of S. luhuanus have been found at stations where
only a few weeks previously the species was absent.

Numerous species of sea cucumbers (Holothuroidea)
are found on sediment — sometimes near reefs, but not
always. They process sediment through their gut and are
relatively immune to predation, probably because of their
toxin (holothurin). Lamberson (1978) found the relatively
large species (up to % m in length) Thelenota anax in
relative abundance at Encwetak at 5 to 30 m depth. This
species was found on lagoon pinnacles and patch reefs, on
sandy bottoms near reefs, and on the vertical slope off the
leeward side near Biken.

Holothurians are important sand processors of reef
areas. Bakus (1963) reported that Holothuria dificilus in-
gested sediment particles up to 2 mm in size, but about
80% were less than 250 microns in diameter. Holothuria
atra fed on even larger rubble, up to 20 mm in size. Bakus
(1973) indicates that beyond selection of suitable size,
there is little specificity among tropical holothurians for
sediments ingested. Hammond (1981) found that among
West Indian holothurians and echinoids (irregular) that sig-
nificant carbonate dissolution and sediment grain-size
modification did not occur during passage of sediment
through the guts of five species of tropical deposit-feeding
cchinoderms. A sirr' r situation probably exists for
Enewetak species.

Irregular urchins are important herbivores of open
sandy areas. Extremely high densities (more than 50 m^^)
of moderately large (more than 30 to 35 mm test length)
species have been observed over large areas. This implies

SUBTIDAL ENVIRONMENTS AND ECOLOGY

127

that a significant amount of algal production must be avail-
able for them to survive for any period. Population size of
irregular urchins seems influenced more by recruitment
success than by food availability (V. S. Frey, unpublished
data).

There are many other organisms living in the sediment
which ultimately make their living from the algal produc-
tion occurring on sediment surfaces or passing over the
sediment. The callianassid crustaceans, mentioned previ-
ously for their bioturbational activities, almost certainly
process prodigeous quantities of sediment to winnow the
organics present on the surface of the sediment grains.
They may additionally exploit the algal fragments which
enter their burrow systems.

On hard substratum, some herbivores at Enewetak live
within a limited area which they maintain as a territory.
Some damselfishes, particularly S(egas(es nigricans, estab-
lish and maintain an "algal lawn" of filamentous algae. The
algal lawn is often found on basal dead parts of the fine

branches of Acropora spp. corals and is strongly defended
against intruding herbivores. This action by S. nigricans is
identical to the western Atlantic Stegasfes planifrons, the
first species for which algal lawn maintenance was
described (Kaufmann, 1977). It is likely that S. nigricans
can kill coral polyps in expanding its algal plot, and large
numbers can significantly damage Acropora spp. corals.
The darkened areas of damselfish algal plots are common
features of Acropora spp. thickets at Enewetak (Fig. 24).
The long-range effect of these areas of dead coral has not
been examined, though areas a few meters square of dead
Acropora are often found in the midst of dense thickets
(Fig. 24). These may potentially represent old algal plots
eroded away by other herbivores.

The general lack of herbivores as significant as fishes
at Enewetak presents an interesting contrast to reefs in
some other areas of the world. In the tropical western
Atlantic, sea urchins, particularly Diadema antillarum, play
a role as herbivores equal to or superior to that of fishes

Fig. 24 Algal "lawns" on Acropora sp. corals produced by the herbivorous damselfish Stegastes nigricans in the Enewetak Island
quarry. Upper left: Large dead area in the Acropora sp. coral possibly produced by the presence of an algal lawn. Upper right:
Aerial view of Enewetak quarry Acropora sp. with many algal lawns (dark spots) established on the coral. Lower left: Acropora
sp. coral in the quarry with algal lawns. Lower right: Stegastes nigricans with its algal lawn (dark area to right of fish).

128

COLIN

(Ogden, 1976). Sea urchins are abundant and conspicuous
elements of the reef fauna. At Enewetak and much of the
Indo-West Pacific, sea urchins are considerably less abun-
dant. Diademnids are particularly less conspicuous, being
small and deeply hidden in the reef. One possible explana-
tion for this difference is a higher population of fishes
which prey on sea urchins in the Indo-West Pacific (Fricke,
1971). in general, western Pacific fish faunas are consider-
ably more diverse and "highly evolved" than the Atlantic,
and more species may be adapted for exploiting sea
urchins (among other things) as food.

Gilmartin (1960) indicated that herbivores have a much
smaller influence on benthic algal communities at 19 to
63 m at Enewetak than they do on shallower communities.
Bakus (1967) felt that grazers influence the benthic biota
most in water <10 m deep.

With heavy grazing pressure from herbivores, the pres-
ence of dense abundant algae implies some reason for its
avoidance by herbivores. For example, the filamentous
strands of the blue-green algae, L[^ngbia sp., occur
extremely abundantly on projections, particularly corals; on
many reefs at depths below 6 to 9 m on windward lagoon
reefs; and as shallow as 1 m in protected areas. The alga
covers large areas of the substratum, streaming from corals
and resembling long reddish hair. Often it virtually covers
the coral with a tangle of filaments that is extremely diffi-
cult to remove. Lf^ngbia sp. often seems to have detrimen-
tal effects on the live coral with the coral tissue beneath
the algae appearing unhealthy. Often coral areas beneath
the alga are dead, but whether the alga is the causative
agent or simply grows on available substratum is not
known. At some lagoon pinnacle reefs, such as Medren
Pinnacle, Liingbia sp. appears to have a significant impact
on the total reef and may be significant in coral mortality
there. Li/ngbia sp. is also abundant on some lagoon margin
patch reefs below 6 m depth but is absent on the shal-
lower portions of the same reefs.

The small sea hare, Sfylocheilus longicauda, occurs
abundantly on the Li^ngbia sp. Sarver (MPRL, 1976)
reported it feeds almost exclusively on Liingbia sp. and
spends its life, exclusive of larval stages, on the alga. The
sea hare accumulates an antitumor agent, debromoaplysia-
toxin, from L^ingbia sp. and Oscillatoria sp. at Enewetak.
This poisonous lipid was first isolated from the digestive
tract of S. longicauda but has its origin from the blue-green
algae (Moore, MPRL, 1976). Sarver found that adult
S. longicauda (about 3 to 7 g) reproduce rapidly, at about
30 days age, and consume about 75 to 100 g of
the alga during their lifetime.

Bioturbation in the Deep Lagoon

A high level of bioturbation in sediment bottoms
throughout the deep lagoon has been verified by recent
work (Suchanek and Colin, 1986; Suchanek et al., 1986).
Gilmartin (1960) first noted, based on in-situ observations,
significant bioturbation of deep lagoon bottoms, but several

other authors have commented on it Emery et al. (1954)
noted, in shallow lagoon photographs, disturbance of the
bottom and burrowing. Hillis Colinvaux (1980) noted a
"relative prominence of animal mounds and castings on the
lagoon floor near the base of pinnacles in 40 m."

Bioturbation of the deep lagoon is evidenced by the
ubiquitous presence of "lebensspuren," a term designating
any sedimentary structure produced by a living organism
(Hantzschcl, 1962). A wide variety of lebensspuren occurs
on sediment bottoms at Enewetak, but the conical mounds
of ghost shrimps (Callianassids) that are as much as 1 m in
diameter and 30 to 40 cm high are the most apparent
type. The conical mounds represent the excurrent open-
ings of complex burrow systems which penetrate deep into
lagoon sediments and underlie nearly all the sediment
bottoms.

Photographs from the Enewetak Lagoon benthic sur-
vey, observations from the submersible Makali'i, and scuba
diving on the lagoon margin have confirmed the near pan-
lagoon (below 10 m depth) distribution of callianassid
mounds. The basic morphology of the burrow system,
pumping rates, and sediment processing have been exam-
ined and will be discussed subsequently. Since the lagoon
sediments are the major repository of remaining radionu-
clides at Enewetak, an understanding of the mixing and
resuspension abilities of lebensspuren-producing organisms,
particularly callianassids, is of basic relevance in any con-
sideration of the future fate of long-lived radionuclides in
the marine environment.

Callianassid mounds are often referred to as "vol-
canoes" because of their conical shape with steeply sloping
sides, a small apical depression (crater), and the resulting
eruption when water and sediment are pumped out of the
apical depression at irregular intervals. These volcanoes
can be so dense that essentially no level substratum can be
found in a large area, the bottom being comprised solely of
volcanoes and incurrent depressions of the callianassid bur-
row systems. It is estimated, based on the photographic
survey, submersible work, and diving observations that
about one volcano per square meter occurs overall in the
lagoon below 10 m depth. Since approximately 85% of the
lagoon bottom is soft substratum and covers about 8 X
10 m , on the order of 10 callianassid volcanoes occur in
the Enewetak Lagoon. Densities may vary from place to
place by a factor of 10, and several species of callianassids
are certainly involved.

A typical callianassid burrow system at Enewetak con-
sists of three major elements: (1) conical depressions on
the surface where sediment enters the system, (2) a com-
plex network of horizontal and vertical burrows, and (3)
conical mounds (volcanoes) where sediment and water are
discharged. The incurrent openings to the system
(Suchanek and Colin, 1986), in which sediment and water
are drawn into the system, appear as a conical depression
many centimeters in diameter. The excurrent side of the
system is represented by the volcanoes, each of which is
fed by a vertical tube at its center through which sand and
water are pumped by the action of ghost shrimp in the

SUBTIDAL ENVIRONMENTS AND ECOLOGY

129

tunnels below. Burrow systems linking the down holes with
the volcanoes are complex, often consisting of a series of
interconnected horizontal tunnels (as much as 4 to 5 cm in
diameter), and sloping to vertical tunnels connecting differ-
ent levels. As much as 1300 g of sediments were ejected
per day from each volcano. Callianassids alter grain size
distribution of processed sediment to produce a very con-
sistent sediment size fraction which is depleted (compared
to some other Enewetak sediments) in both coarse (>2
mm) and fine (<90 microns) sediments (Suchanek and
Colin, 1985). As much as 3 liters of water evolves daily
from volcanoes during sediment-pumping activities (Colin
et al., 1986). Volcano water contained suspended particu-
lates >0.45 microns in diameter at levels at least five
times that of water immediately overlying the sediment,
which itself has elevated particulates compared to "aver-
age" lagoon water (Colin et al., 1986).

"Tagged" (painted with fluorescent paint) sediment
experiments have demonstrated that most large particles,
more than 1.5 to 2 mm in diameter, entering the ghost
shrimp burrow system are not returned to the surface
(Suchanek et al., 1986). Probably such particles are too
large to be temporarily suspended by pumping, and it is
believed that callianassids "store" large particulates in
unused portions of the burrow system.

A constant "disturbance" effect occurs in bioturbation
at Enewetak. The constant grazing by herbivores, the
digging into the sediment by carnivores looking for prey,
and the ingesting and then expelling of sediment by the
surface-dwelling species cause the upper few centimeters
of sediment to be constantly disturbed (Suchanek and
Colin, 1986). The sediment surface, through this action, is
a continuous mosaic of small pits and mounds, disturbed
places and tracks, all from this surface reworking. In the
lagoon at depths shallower than about 5 to 6 m, this evi-
dence is quickly obliterated by wave ripples, but below
that depth the disturbances remain apparent for some
time.

FISH COMMUNITIES

The fish fauna of Enewetak is quite diverse, numbering
more than 800 species. There are, certainly, a number of
species yet to be recorded from the atoll. Fish species are
not evenly distributed around the atoll; many occur com-
monly in only one type of habitat. These preferences result
in general fish communities which are identifiable assem-
blages of species. Hiatt and Strasburg (1960) made the
first (and still best) attempt to characterize fish communi-
ties of various areas of Enewetak and to examine their
trophic relationships. Subsequent researchers have exam-
ined feeding by various portions of the Enewetak fish
fauna (Hobson and Chess, 1978; Randall, 1980; Bakus,
1967; Gerber and Marshall, 1974; Smith and Paulson,
1974; Reese, 1975, 1977; and others), but a definitive
study of the overall trophic dynamics of fishes has never
been undertaken.

Randall has reviewed records of fishes since Schultz
and collaborators (1953 to 1966), and a checklist is
included in Chapter 27 of Volume 11, this publication.
Although this may be approaching a definitive list of fishes
for the Marshall Islands for shallow-water species, it was
apparent from the observations and photographs from the
submersible Makali'i that a significant number remain to be
recorded (many undoubtedly undescribed) from depths
greater than those usually penetrated by scuba divers.

The species of fishes inhabiting a given location at
Enewetak are strongly influenced by environmental factors.
Primary among these are substratum types (hard or soft,
variations of these), depth, current, wave action, and oth-
ers. The food of Enewetak fishes is based on two different
sources: primary production from atoll bottoms and waters
and oceanic zooplankton and phytoplankton. The relative
importance of these two pathways has never been
rigorously compared, but the high productivity of reef flat
and lagoon versus the low density of phytoplankton and
zooplankton in oceanic water upcurrent of Enewetak imply
that the former is of considerable significance. Seeing the
immense numbers of large, herbivorous fishes on spur and
groove, reef flat, and shallow patch reefs impresses one
with the amount of fish life supported by algae growing on
the substratum.

Predators of mid- and upper-water lagoon areas arc
varied. Hiatt and Strasburg (1960) differentiate mid- and
surface-water communities but p>oint out that some large
carnivores enter both areas. They felt that surface water
communities contained various sizes of zooplankton, small
plankton-feeding fishes (round herring and silversides),
larger macroplankton-feeding fishes (such as halfbeaks),
and piscivores (needlefish, tunas, barracuda, jacks). Randall
(1980) and Hiatt and Strasburg (1960) have discussed the
food habits and general habits of many of these.

The Carangidae are important predators at Enewetak.
The members of the genus Caranx are largely fish eaters,
occasionally taking cephalopods or crustaceans. Caranx
ignobi/is, the largest species of the genus, reaches 80 kg,
and as Randall (1980) notes "may be encountered any-
where in the atoll environment including water surprisingly
shallow for such a large fish." Caranx melampi^gus is very
common on Enewetak reefs and feeds largely on reef
fishes, including some such as Caracanthus sp., that live
deep within the branches of living corals. The rainbow
runner, Elagatts bipinnulatus, is a mid-water feeding caran-
gid which occurs in schools above reefs.

Among scombrid fishes, the dogtooth tuna,
G\;mnosarcla unicolor. is the only tuna commonly seen
around lagoon pinnacles. It also occurs on outer reefs and
is a predator on free-swimming fishes, including Naso
spp., Caesio, Ptercxaesio, and Decapterus (Randall, 1980).
Hiatt and Strasburg (1960) reported it and the common lit-
tle tunny, Euthynnus affinis, as slashing through the dense
schools of round herring.

Some species of moderately large fishes are detri-
tivores. Mullets are common on shallow reefs, both lagoon-
ward and seaward of the reef flat. Crenimugil crenilabis has

130

COLIN

been seen to expel sand through the gills after feeding, a
process in eommon with many smaller fishes, and appears
to feed on fine algae detritus (Randall, 1980).

The largest planktivorous bony fish at Enewetak is
probably the milkfish, Chanos chanos, which occurs occa-
sionally on outer reefs and in the lagoon. The largest
planktivore, at least within the lagoon, is the manta ray,
Manta alfredi. They frequent lagoon margin areas, often in
water 3 to 6 m deep, and the wide channel area. On one
occasion, a group of more than 100 M. alfredi with a 2 m
or more span were seen from the air in the deep ocean
just beyond the wide channel.

Randall (1980) has summarized the food habits of
larger groupers (Serranidae), snappers (Lutjanidae), and
emperors (Lcthrinidae). All are benthic predators, although
some groupers and snappers will rise to a lure in mid-
waters.

Large oceanic predatory fishes occur commonly around
Enewetak. Tunas, wahoo (Acanthoc\,;bium solanderi),
dolphin (Cor^/phaena hippurus), and billfishes are known to
frequent waters within a few kilometers of shore (Schultz
et al., 1952). Hiatt and Strasburg (1960) note that the
presence of larval fishes and crustaceans produced by reef-
and shore-dwelling adults, "supplementing the usual high
seas forage species, probably is significant in attracting
tunas (and other large pelagic fishes) to mid-ocean islands."

There are numerous fishes that are highly specialized
in their food habits. For example, in the Chaetodontidae,
Reese (1975, 1977, 1981) found that at Enewetak, 10 of
17 species are coral-feeders, whereas two are planktivores
and five are "omnivores " Among coral feeders, four were
believed to be obligate coral predators, with fine comb-like
teeth for biting off coral polyps. One species, Chaetodon
ornatissimus, appears to eat coral mucus with its fleshy
lips rather than biting off the polyps like other species.
Other coral-feeding species at Enewetak ingested other ani-
mal matter as food. The other extreme is C unimacuhtus
which even ingests fragments of septa as it feeds on
polyps.

Other coral-polyp feeders include Oxiimonocanthus lon-
girostris, Labnchth\js unilineata, and Labropsis spp. Some
damselfishes, such as Plectrogi;lphidodon johnstonianus and
P dickii, have been observed to feed on coral polyps (Ran-
dall, 1974).

Few Enewetak fishes feed on sfjonges. Hiatt and Stras-
burg (1960) recorded only Arothron mappa, a puffer, as
having eaten sponges. They examined, however, only one
species of Pomacanthidae, a group shown to contain
sponge-feeding species in the West Indies (Randall and
Hartmann, 1967).

Among fishes there are several "cleaners" at
Enewetak, those species which eat ectoparasites and con-
sume mucus from the bodies of other, usually larger,
fishes. Most important are members of the wrasse genus
Labroides, particularly L. dimidiatus. There are also various
invertebrate cleaners, usually shrimps, on Enewetak reefs.

Some fishes associate with invertebrates that are
avoided by predators as one method of gaining protection.

Anemonefishes associate with sea anemones; in spite of
this, they are occasionally eaten by other fishes. Hiatt and
Strasburg (1960) found a juvenile Amphiprion melanopus
in Apogon noLiem/asciafus. Allen (1972a) reported that
disoriented Amphiprion (due to "fin-clipping" manipula-
tions) were sometimes eaten by groupers, particularly
An[^perodon leucogrammicus. Allen (1972b) described a
cardinalfish, Siphamia fuscolineata, sheltering among the
venomous spines of the crown-of-thorns starfish,
Acanthaster planci. Between eight and 31 fish were found
with each of four A. planci; however, only a small percent-
age of starfish had the apogonid associated with it. Species
of Siphamia are more often found associated with diadema-
tid sea urchins.

A small group of fishes shelter among branched corals,
some never leaving the coral. Hiatt and Strasburg (1960)
illustrate some which include the gobies of Gobiodon and
Paragobiodon , plus the scorpaenoid genus Caracanthus.
There are similar invertebrate associates, particularly crabs
of the genus Trapezia and some alpheid shrimps. A much
greater number of fishes temporarily shelter in branched
corals when danger threatens. The hundreds of Chromis
caerulea. C atripectoralis. Dasc^illus reticulatus, and
D aruanus stationed above small heads of Pocillopora
corals which can take nearly instant shelter on that head
(Hobson and Chess, 1978) are astounding.

Hobson and Chess (1978) have examined the feeding
relationship between zooplankton and planktivorous fishes
of the lagoon margin. At their two study areas, one
northeast of Jedrol within the strong influence of currents
in the deep channel and the second in the lee of Bokandre-
tok where currents are weak, they found that current pat-
terns sharply affected trophic relationships. The plankters
ingested by diurnal and nocturnal planktivores were quite
different. There was an abundance of suitable zooplankton
in strong current areas, whereas areas of weak currents
were poor in zooplankton. These poor areas in the lee of
reefs and islands were, however, rich in debris from the
reefs and, among diurnal planktivores, many fishes here
were adapted to feeding on algal fragments. Some species,
common in both strong and weak current areas, showed a
shift in diet between areas reflecting the type of food items
in the water column.

Nocturnally, fish planktivores were more abundant in
weak current areas feeding on larger zooplankton which is
absent from the water column during the day. Much of this
zooplankton shelters on or in the substrate during the day,
rising into the water column at night. During the day many
nocturnal planktivores shelter on reefs. Horch (1973)
found both M\jripristis violaceus and M. pralinius common
in shallow water during the day, coexisting in coral caves
of patch reefs and reefs fringing some islands. At night
they left their shelters and often fed in mid-water within a
meter of the water's surface.

Hobson and Chess (1978) found a clear-cut differentia-
tion in the distance that various planktivorous reef fishes
move away from reef shelter to feed in the water column.
On windward lagoon margin patch reefs, they found that

SUBTIDAL ENVIRONMENTS AND ECOLOGY

131

species stationed farther from the reef had more cylindrical
bodies with deeply incised caudal fins than species remain-
ing relatively close.

The fish communities on lagoon margin patch reefs
were examined in detail by Nolan (1975) for reefs between
Enewetak and Medren. These reefs are typical of those
found throughout the lagoon margin on the windward side.
He divided the fish community into four assemblages:

1 . The patch reef assemblage (about 25 species)

2. The roving fish assemblage (about 25 species)

3. The sand assemblage

4. The rubble assemblage (3 and 4 together about
50 species)

The most numerous fishes living on the patch reefs were
cardinalfishes (Apogonidae) and damselfishes (Pomacentri-
dae) A large percentage of these individuals are mid-water
plankton feeders, relying on items brought by the steady
cross reef flat currents from ocean to lagoon. Nolan (1975)
found that fish species composition of lagoon margin patch
reefs on the windward side visually censused at about
100-day intervals fluctuated considerably over 2'/2 years of
observations. Among five "control reefs," each had 20 to
24 species at the end (mean 21). Individual reefs varied by
as many as 10 species during the study. The numbers of
individuals, however, varied by as much as a factor of 10
during the study. One reef went from about 100 individu-
als to 970 because of juvenile recruitment of two species
of apogonids and pomacentrids. If increases related to
juvenile recruitment were not considered (or in the case of
those reefs where massive juvenile recruitment did not
occur), numbers of individuals were much more consistent,
varying by less than a factor of 2.

Nolan (1975) found considerable movement among reef
fishes between lagoon margin patch reefs on the windward
side of Enewetak. He reported various surgeonfishes,
wrasses, and parrot fishes as ranging freely between reefs.

Although identifiable assemblages oi' fishes occur in a
particular environment, there is small-scale variation in spe-
cies composition. Nolan (1975) constructed artificial reefs,
made of cement pipe "modules," on the lagoon margin
between Enewetak and Medren to provide identical shelter
to reef fishes which inhabited those reefs. Artificial reefs
reached species equilibrium in 100 to 200 days, a figure
equivalent to defaunated natural reefs, but the colo-
nization pattern differed from natural reefs. About 10 spe-
cies occurred on the artificial reefs (versus about 20 for
small natural reefs), which had limited habitat diversity,
and variation over time was much higher for artificial reefs
than natural reefs.

Gladfelter et al. (1980) examined the fish communities
of lagoon margin patch reefs between Enewetak and
Medren and near the deep channel but utilized reefs over
an order of magnitude larger than those studied by Nolan
(1975), averaging 150 to 200 m^ in area. Compared to
western Atlantic patch reefs of similar size, Enewetak reefs
are steef)€r sided with greater vertical relief and more com-
plex surface topography. The number of species (visually

censused) on the Enewetak patch reef varied between 76
and 109, with about 500 to 900 individuals per reef. Con-
sidering trophic categories, diurnal planktivores were more
abundant on Enewetak than Virgin Island patch reefs,
probably because of the consistent ocean to lagoon cross-
reef currents. Herbivores were more diverse among
Enewetak reefs with fewer individuals. The Virgin Islands
reefs were surrounded by sea grass beds, a habitat lacking
at Enewetak, and had more nocturnally foraging
invertebrate-feeding fishes.

The interrelationships between reef fishes on Enewetak
patch reefs are complex. Competition for space and
food can be intense between species and among conspecif-
ics. Factors controlling initial recruitment of juveniles and
their eventual growth to adults are additional controllers of
ultimate community composition. Nolan (1975) describes
numerous instances of unique interactions among fish
species inhabiting small patch reefs on the lagoon margin.
Many of these interactions were the result of experimental
manipulation, but others were simply the result of long-
term careful observation of the environment. Nolan's
(1975) record is, perhaps, the best such record of relation-
ships and occurrences among a diverse group of fishes on
small reefs over time.

Allen (1972a, b) described instances where removal of
adult anemonefish from their host anemones was followed
within a few weeks or months by recruitment of large
numbers of juvenile Amphiphon. Anemones unoccupied by
Amphiprion were not encountered by Allen (1972a, b),
and he felt that anemone availability was one of the major
factors limiting anemonefish populations at Enewetak. The
situation has not changed since Allen's work; anemones
remain relatively uncommon and Amphiphon populations
are limited compared to other tropical Pacific areas.

The fish communities of the outer reefs, deep lagoon,
and open ocean around Enewetak are not as well docu-
mented. Even the nearshore spur and groove is poorly
known because of its normally hazardous surf conditions.
The movement of herbivores and predators onto the reef
flat with rising tides is well documented (Hiatt and Stras-
burg, 1960; and others). Population sizes, movement along
the reef face, and foraging dynamics are not well known.

FISH REPRODUCTION AND
RECRUITMENT

Most reef fishes reproduce by either laying demersal
eggs on the substrate or releasing planktonic eggs in the
water column. Both hatch as planktonic larvae. Larval life
ranges from a minimum of 2 to 3 weeks to as much as 2
to 3 months. Lack of proper substrate for metamorphosis
may greatly extend this time. Some information exists con-
cerning the spawning habits of demersal- and planktonic-
egged species at Enewetak.

Major families producing demersal eggs include p>oma-
centrids, gobiids, and blennies. Swerdloff (1970) and Allen
(1972a, 1975) have described various aspects of pomacen-
trid spawning at Enewetak.

132

COLIN

Most of the larger fishes at Enewetak produce plank-
tonic eggs. Relatively little has been published about the
spawning of larger fishes at Enewetak. What has been writ-
ten is limited to the papers by Helfrich and Allen (1975),
Thresher (1982), and Bell and Colin (1986). There are
considerable unpublished data of Colin and Bell. Spawning
habits of about 60 species are known and, although gen-
eral patterns are known for these, there are exceptions to
every generalization.

Many planktonic-egged species can spawn at any time
during the day in certain locations when tidal conditions
are correct. This is generally true for the labrids and parrot
fishes, but other families, such as the Pomacanthidae, are
believed to spawn only near sunset (Thresher, 1982; Bell
and Colin, 1986). In spite of the abundance of large pis-
civores, predation on spawning fishes appears to be rare.
Predation on eggs immediately after release by particulate
plankton-feeding fishes is also uncommon, occurring in only
a few percent of spawning releases.

Planktonic eggs and larvae from both demersal and
planktonic eggs are carried by currents during their
development. Larvae produced on the windward side of
the lagoon, particularly the northern part, would have an
excellent chance of undergoing their entire development
within the lagoon, since water residence times in that area
are above the mean of about 30 days, reaching as much
as 4 months. The mid-depth water return mechanisms of
the lagoon would ensure return of larvae to the windward
side in spite of the westward surface drift.

There is no distinct seasonality known in spawning of
fishes at Enewetak, but relatively small differences cannot
be ruled out. Gerber (1981) found approximately a two-
fold increase in the mean number of fish eggs in mid-
lagoon plankton tows during summer as opposed to winter
periods, but considerable variability in individual collections
indicated different means were not significant. Given the
transitory nature of fish spawning, the observed patchiness
of eggs is not surprising. A similar situation existed for fish
larvae (Gerber, 1981). Higher concentrations of fish eggs
and larvae at significant levels were found at Gerber's
(1981) "behind reef" station than in the mid-lagoon during
winter and may be the result of distance from sources
(reefs and their immediate vicinity) of eggs and larvae.
Other larvae are undoubtedly carried out to sea, but their
potential fate is not well known. The presence of down-
current eddies (in this case to the west) behind islands (and
atolls) is well documented and may serve to return larvae
to the vicinity of Enewetak after a p)eric)d of days or
weeks. More work is needed on this phenomenon. Many
larvae are certainly lost into the general westward drift of
the North Equatorial Current, but sufficient numbers of lar-
vae develop within the lagoon or are returned by eddies to
maintain fish populations at the atoll. A limited number of
recruits must originate east of Enewetak, from Bikini and
other atolls of the northern Marshalls, but in terms of
numbers are probably overwhelmed by locally produced
offspring.

Nearly all Enewetak fishes recruit as free-swimming lar-
v.Tse. Exceptions would include elasmobranchs (sharks, rays)
bearing live young and a limited number of reef fishes
which have live young (Brotulidae, Ophidiidae) or a greatly
modified larval life (Syngnathidae). Most of the reef fishes
have planktonic larvae which must make a transition when
becoming juveniles, often moving into a reef environment
crowded with others of their species and other species.

There was no significant evidence for seasonality of
reef fish recruitment to artificial reefs in Nolan's (1975)
study. Some species, however, did not recruit at all sea-
sons. Possibly, this was because of the relative scarcity of
those species, but one common species Apogon 'nouae-
guinae" (the species identified as nouaeguinea by Lachner,
Schultz, and collaborators, 1953, appears to be A. c^iano-
soma though seemingly subspccifically different) did not
recruit during the summer. Since it did appear in small
numbers on natural reefs, Nolan (1975) attributed this,
potentially, to reduced recruitment during the summer.

Year-round spawning activity and reproductive colora-
tion were observed in some apogonids and pomacentrids.
Female chaetodontids with ripe ovaries were noted at all
seasons by E. S. Reese (personal communication).

The role of predators in limiting the numbers of some
small reef fishes on patch reefs has been amply demon-
strated by Nolan (1975). He found that when additioijal
pomacentrids {Chromis and Dascy//us) were added to reefs
already at saturation levels with conspecifics, the new
arrivals were readily eaten by cruising piscivores. One
artificial reef already at equilibrium had additional damsel-
fishes added. Within a day or two almost all additions had
perished. Shelter is a factor which limits absolute numbers
of such reef fishes; the excess individuals which cannot
find a refuge are easily taken by the abundant predators of
Enewetak reefs.

CIGUATERA

Ciguatera is the most common tropical fish jjoisoning
known in the Marshall Islands, including Enewetak. Randall
(1980) has reviewed the historical reports of ciguatera in
the Marshall Islands. Of relevance was information pro-
vided by Iroij Johannes Peter that before 1946, some reef
fishes from certain areas of Enewetak were pxDisonous to
eat. Randall (1980) described instances of ciguatera poison-
ing at Enewetak. The internal organs (which are consider-
ably more toxic than the flesh) of 47 species of large reef
fishes were tested using a mongoose bioassay for toxicity.
At least one individual of five species produced the strong-
est reaction (death within 48 hours), whereas 31 species
produced at least some response by mongooses to inges-
tion. Even in the species producing the most frequent reac-
tion, the percentage of individuals producing a response is
relatively small. Ciguatoxic fishes at Enewetak were found
to fit the recognized pattern of being generally large indi-
viduals, mostly roving predators, and largely piscivorous
(Randall, 1980). No evidence exists that the occurrence of

SUBTIDAL ENVIRONMENTS AND ECOLOGY

133

ciguatera at Enewetak is related to radiation in the
environment. Disturbance of the marine environment
(dredging, construction, wrecks, etc.) has been strongly
implicated in producing ciguatera (Randall, 1980).

The probable causative organism of ciguatera, a
dinoflagellate Gambierdiscus tox'cus, has been identified
and the toxin collected and purified.

Sharks

Several species of sharks are common at Enewetak.
They range from nearly harmless to extremely dangerous.
Some are found in only one environment, whereas others
are nearly ubiquitous

The blacktip reef shark, Carcharhinus melanopterus, is
abundant on the reef flats all around Enewetak. Hiatt and
Strasburg (1960) reported C. melanopterus was the most
common shark on windward and leeward reefs. Hobson
(1963) reported blacktip sharks were most often observed
on sand and coral rubble flats in shallow water. It often
penetrates into water so shallow the dorsal fin and back
are well exposed. Small C melanopterus individuals are
most common on the reef flat. Larger individuals cruise
the spur and groove zone offshore and are often seen
around lagoon margin pinnacles.

The whitetip reef shark, Triaenodon obesus, is f)€rhaps
the next most commonly seen species. It is most abundant
in the lagoon along the marginal sandy areas and reefs but
is also found on seaward reefs. Hobson (1963) found
T. obesus most often on patch reefs and coral ledges
around the margin of the lagoon. Randall (1977) reported
that T. obesus feeds largely on reef fishes, especially
scarids and acanthurids, plus octopuses.

Also found in the lagoon is the lemon shark, Negaprion
brevirosths, which although large, penetrates into relatively
shallow water. The author once nearly stepped down onto
the back of a 1.5 m lemon shark while wading ashore on
Ikuren in knee-deep water.

The most studied and the most dangerous shark at
Enewetak is the gray reef shark, Carcharhinus ambli;rh\;n-
chos. It is found throughout the lagoon and on the sea-
ward reefs. Hobson (1963) felt it was most abundant in
the deeper waters of the lagoon and passes. Attacks on
humans are discussed subsequently. Johnson and Nelson
(1973) described in detail the threat display of the gray
reef shark, which often precedes an attack. Sharks placed
in a situation of a diver potentially restricting its escape
produced the most intense displays: an exaggerated, often
rolling, swimming motion with back arched, pectoral fins
dropped, and snout lifted. Starck (MPRL, 1971 to 1972)
elicited attack responses on a small wet submersible by
pursuing C ambl\;rh\^nchos. A more detailed account is
presented in Anderson (1980). Subsequently, this attack
and its preceding threat display have been investigated by
Nelson (MPRL, 1978, 1979). He found that the shark usu-
ally attacks after displaying if the object or person contin-
ues to approach. The attacks are sudden, high-speed

strikes, often with the mouth open. He believed that
"oriented pursuit" by the small submarine was of primary
importance in releasing an attack. A straight-line pass near
the shark never released an attack, although it did produce
the threat display. For more information on gray reef
shark behavior, see Nelson et al. (1986).

Randall (1980) reported that C. ambliirht;nchos from
Enewetak and other localities fed mostly on reef fishes
and, to a lesser extent, on cephalofxxls. It is the most
common shark seen on seaward reefs. Off the southwest
islands and on the leeward reef face, it usually app>ears
before the silvertip shark, C. albimarginatus, and out-
numbers the latter shark two or three to one. In the
lagoon it is common around mid-lagoon pinnacles where it
seems particularly aggressive. Often when a boat stops in
mid-lagoon on a calm day, one or more C. ambl^rhi^nchos
will rise to investigate the boat from water 50 to 60 m
deep.

The movements of C ambl\^rhi>nchos tagged with ultra-
sonic transmitters have been investigated by Nelson
(MPRL, 1978, 1979). He has determined that gray reef
sharks often move surprisingly long distances around
Enewetak. Deep-water gray reef sharks tagged on or near
the drop-off of the seaward reefs ranged as much as
16 km along the reef in one night. They were not as
predictably home ranging as lagoon gray reef sharks, and
Nelson (MPRL, 1979) suggested they might represent a
more nomadic segment of the population. Lagoon grays
were tracked for as long as 21 days, and although some
stayed in one area, others moved considerably. One
individual tagged at the mid-lagoon "dome" pinnacle sfsent
the daylight hours near that pinnacle but ranged widely at
night. Its home range was estimated at about 53 km .
McKibben and Nelson (1986) discussed movements of
tagged gray reef sharks at Enewetak.

Other seaward reef sharks are the silvertip shark,
Carcharhinus albimarginatus, and Galapagos shark,
C galapagensis. The silvertip shark is found normally on
seaward slopes below 20 to 30 m, although Randall
(1980) observed one individual in the lagoon in water 2 m
deep. There are reports of C. albimarginatus as deep as
400 m (Randall, 1980). Silvertip sharks feed almost
exclusively on fishes, both reef and open water. Randall
(1980) also found at Enewetak a gray reef shark over 60
cm in total length in the stomach of a C. albimarginatus
that was 1.6 m in total length. The Galapagos shark is a
large, dangerous species, but fortunately it is uncommon at
Enewetak. Randall (1980) collected only a single specimen,
but little is known of its habits beyond feeding on fishes
(including sharks) and cephalopods.

Probably the largest dangerous shark in Enewetak
waters is the tiger shark, Galecerdo cuvier. Randall (1980)
examined two specimens from Enewetak, 1.7 and 2.4 m
precaudal length (length minus the caudal fin), of 72 and
174 kg, respectively. McNair (1975), an accurate and
experienced shark observer, while diving on the leeward
seaward reef, observed a huge tiger shark pass above him

134

COLIN

which he estimated was longer than the 21 -ft boat it
passed by. Tiger sharks are seldom seen by divers and,
therefore, are not as much of a hazard as some smaller,
dangerous species. Randall (1980) found the scutes of a
green turtle shell, shark vertebrae, bird feathers, digested
shark fins, and pieces of a porpoise in the stomachs of
Enewetak G. cuvier.

There have been several instances of shark attacks at
Enewetak. Most have involved the gray reef shark and, in
some, injury occurred to the human involved. Hobson et
al. (1961) documented two incidents with gray reef sharks
in which spearfishing probably stimulated aggressive
behavior. Fortunately neither instance resulted in injuries.
Not so lucky was another individual whose head was
slashed by the upper jaw of C. ambl\;rh[jnchos after the
powerhead he was using to try to kill this shark failed to
detonate on impact (Randall, 1980).

In April 1978, another attack by C. ambli/rhi;nchos
occurred in which a 1.5-m (5-ft) long individual severely
mauled the right arm of a diver and attacked his diving
partner (M. V. deGruy and P. Light, unpublished report).
In this case deGruy approached the shark, which was ex-
hibiting the threat posture, in an attempt to photograph it.
When deGruy triggered the electronic strobe of his cam-
era, the shark turned, rushed toward deGruy, and seized
his arm. Seconds later the shark tore a chunk from one
diving fin. As the diver's companion, Light, swam to his
assistance, the shark bolted toward Light and badly
slashed his hand. The shark disappeared. Both divers sub-
sequently recovered from their wounds.

The most recent attack by C. ambli^rhimchos on
humans occurred in January 1982 when one of the repa-
triated Marshallesc. while spearfishing, had his left arm
mauled by what was probably a gray reef shark. Several
sharks were around this fisherman and his two compan-
ions, who were carrying a considerable quantity of dead
fishes.

Randall and Helfman (1973) reported two instances of
C. melanopterus menacing humans at Enewetak.

It is interesting to note that despite repeated success in
producing threat displays and attacks by C. ambli;rhi;nchos
by pursuit with small wet submersibles, similar attempts
have failed to produce the threat or attack by C albimar-
ginatus, C. melanopterus, and T. obesus (Nelson, MPRL,
1979; Starck, MPRL, 1971 to 1972).

Crater Life

Nolan et al. (1975) described bottom substrata and
fishes inhabiting the two small craters (Cactus and
Lacrosse) at the north end of Runit Island. Hard substrata
were restricted to the upper 4 m and the sides of both
craters sloped quickly to a sediment plain at about 12 m
deep. The bottoms of the craters were "heavily excavated
by several species of gobies and burrowing shrimps."
Other bioturbating organisms were also present. Colonies
of Hahmeda and Derbesia minima were abundant on the
sediment bottom. Hillis-Colinvaux (1980) found no

Halimedae in Lacrosse crater but found a pure, dense
strand of Halimeda incrassata in the murky center of
Cactus crater at 11 m depth. This merged peripherally
with Caulerpa ad serrulata. but no loose plants were seen
on the sides of the crater. Halimeda incrassata was rare at
Enewetak (Hillis-Colinvaux, 1980), and the Cactus crater
population was the only dense (about 200 or more thalli
m~^) strand found at Enewetak. Hillis-Colinvaux (1980)
suggested that the extremely soft and fine sediment of
Cactus crater might have promoted the growth of this
dense strand, possibly from a limited number of vegetative
propagules.

Nolan et al. (1975) found little living coral in the Runit
craters but reported that "molluscs, crustaceans, poly-
chaetes, zooplankton, algae, and phytoplankton found in
the craters seemed typical of the fauna and flora occurring
in the adjacent lagoon or upon the reef flat." Eighty-four
species of fish were observed or collected in the craters;
the number is incomplete for various cryptic families. Cen-
suses at near high and low tides indicate fewer individuals
of species at low tides than at high tides.

The third small atomic test crater, Seminole, on Bokcn
(North) Island has not been examined biologically. Adjacent
sand flats seem to be an area with high numbers of small
blacktip sharks. Circulation into Seminole crater is much
more restricted than circulation into either Runit atomic
crater.

The three large thermonuclear craters in the north
lagoon have not previously been described biologically.
During the Enewetak Submersible Project, several dives
were made in Oak crater using the submarine, and addi-
tional scuba dives were made on the crater slope. The
level bottom of Oak crater (Ristvet et al., 1978; Chapter
4, this volume) was heavily bioturbated, almost certainly
by callianassids, with a mound density equaling that
observed anywhere else at Enewetak. In addition, the
irregular urchin Maretia planulata occurred in high densities
of 10 m on the surface of sediment at the bottom of the
crater. Similar bioturbation was evident in Koa crater,
although at a lesser depth. Nelson and Noshkin (1973) did
not consider that biologically mediated presentation of
radionuclides from within the sediment column to the
water column was occurring in Koa (and Mike) craters but
that the "principal loss of activity from the deposits may
only be from the slow release to the overlying waters."

Smith and Brock (MPRL, 1976) found that the Mike-
Koa and Oak craters have large amounts of rubble in the
vicinity of the craters which provide an unfavorable sub-
stratum for coral growth. The locations of the craters are
described in Chapter 3 of this volume.

Why Are There No Sea Grasses
at Enewetak?

Tsuda et al. (1977) have summarized the known distri-
bution of sea grasses in Micronesia. Only one species,
Thalassia hemprichii, is known from the Marshall Islands.
Records exist for it from Ailinglapalap, Jaluit, and Ujilang

SUBTIDAL ENVIRONMENTS AND ECOLOGY

135

Atolls. At the last atoll, Fosburg (1955) reported "a rather
extensive strip" of T hemprichii along the lagoon shore of
Ujilang Island.

Thalassia hemprichii was found by the author only
along the lagoon shore of Ujilang Island in July 1982.
None was seen on several lagoon shores on the windward
side visited or on two coral pinnacles with extensive sandy
areas above 15 to 20 m depth. The strip along Ujilang
Island occurred only at depths <1 m. Reliable information
exists that 7 hemprichii (or any other sea grass) does not
occur at Bikini, Rongelap, or Rongerik (Emery ct al.,
1954). Fosburg (1955) reported on visits to many north
Marshall atolls (Kwajalein, Lae, Ujae, Wotho, Likiep,
Aihik, Bikar, Pokak. and Ujilang) with Ujilang the only of
these where sea grasses were noted. At Kwajalein, signifi-
cant diving and collecting activities by knowledgeable
marine biologists over large portions of the atoll have
failed to reveal 7. hemprichii. Tsuda (Chapter 1, Volume
II) documents that no record of sea grasses from Enewetak
exists. Because of the large amount of marine work carried
out at this atoll, it is reasonable to say they do not occur
here.

The presence of T. hemprichii at Ujilang, only 200 km
away, is intriguing. The decline, however, in the numbers
of sea grass species eastward through Micronesia (Tsuda et
al., 1977) and the probable absence of T. hemprichii at
most — if not all — other Marshall atolls, indicate that per-
haps there has been no opportunity for transport of
T. hemprichii to Enewetak. The areas upcurrent of the
atoll are similar atolls without sea grasses. A similar condi-
tion has been noted for Sargassum (Tsuda, 1976) with no
records from any northern Marshall atoll, including
Enewetak. The means of dispersal of 7. hemprichii are lim-
ited. Potentially it could be transported as drift material
torn loose during storms or as drifting seeds or seed cap-
sules. Both potential mechanisms are current dependent,
which would work against transport to Enewetak. Atolls
farther south, in the influence of the Equatorial Counter-
current, may have received their populations via this cur-
rent.

Zoogeographic Considerations

One interesting phenomenon is that many marine
animals that arc relatively scarce at Enewetak are much
more common elsewhere. This seems true even within the
Marshall Islands where disparity exists between Enewetak
and the more southerly Marshall Island atolls— such as
Kwajalein, Majuro, and Arno — which have had a signifi-
cant collecting effort.

This scarcity is true among fishes. For example, Allen
(1972a) commented that most anemone species were rela-
tively scarce at Enewetak as compared to his personal
observations in Tahiti and literature from the Nicobor
Islands. "Several hours of intensive searching may at best
result in finding four or five widely scattered sp>ecimens (of
anemones) of the variety which harbor Amphiprion." As
has been pointed out elsewhere, even when present, the
occurrence of anemones may be transitory.

Hiatt and Strasburg (1960) indicate that a number of
fish species are uncommon or rare at Enewetak (and often
Bikini) compared to Arno Atoll. They felt that Arno was a
more productive atoll than either Enewetak or Bikini
because it is located in an area of upwelling where the
North Equatorial Current and Equatorial Countercurrent
meet and because it has a higher rainfall than the other
two atolls. Whether this has an effect on the abundance of
reef fishes or whether the differences observed are pro-
duced by sources of larval recruits, etc., is not known.
Species of Plesiops and Pseudogramma are among fishes
that are less common at Enewetak. Randall (1986) lists a
number of species from Kwajalein Atoll which, in spite of
comparable collecting effort, are not known from
Enewetak.

Within the overall picture of Indo-West Pacific shore
fish distribution, the Enewetak fauna is less diverse than
the "core" areas of the Indo-Malayan Archipelago. This is
well known for individual families (Allen, 1975), but the
Enewetak fish fauna is at a level of diversity "expected"
when compared to adjacent areas. The differences exist
with respect to abundance of quite a number of species.

Why Are There No Mangroves at Enewetak? ACKNOWLEDGMENTS

Wiens (1962) has summarized much of the information
on mangroves on atolls. He cites records of "mangroves"
on several southern Marshall Island atolls. Hatheway
(1953) described stands of Sonrieratia caseolahs and
Bruguiera conjugata on Arno Atoll. Wiens (1956) observed
a tidal inlet on Ailinglapalap Atoll with three species of
mangroves. Fosburg (1955) reports B. conjugata on
Utirik, Ailuk, and Lae to be rare and limited to "wet
depressions." Otherwise, he does not record any
"mangrove" species, particularly those of the Rhizophori-
dae from the northern Marshall Islands.

Again the situation is similar to sea grasses. Mangroves
can certainly survive at atolls like Enewetak, but it is likely
their transport mechanisms have never allowed their intro-
duction.

The staff of the Mid-Pacific Research Laboratory
(MPRL) and its predecessor institutions made possible the
vast majority of the marine research undertaken at
Enewetak Atoll since 1954. Many of the people involved
in this work have been cited in the preface to this volume.
I would like to thank in particular the following MPRL staff
members for their help in my own fieldwork and that of
others: L. J. Bell, L. M. Boucher, V. S. Frey, S. Johnson,
J. T. Harrison, III, and R. M. Richmond. The operation of
MPRL would not have been possible without their dedica-
tion and perseverance under extremely difficult cir-
cumstances.

I would like to thank the following for their comments
on the manuscript: L. J. Bell, J. T. Harrison, III, A. Kohn,
and J. E. Randall.

136

COLIN

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138

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Chapter 8

Intertidal Ecologv; of Enewetak Atoll

ALAN J. KOHN

Department of Zoology. (Jniuersify of Washington
Seattle. Washington 98195

INTRODUCTION: THE INTERTIDAL
ENVIRONMENT

At an atoll, land and sea meet at a precarious,
dynamic interface that often bears evidence of horizontal
and vertical movements over past centuries and millennia.
In the shorter-term dynamics of ecological time, the inter-
tidal region of an atoll exposes the plants and animals that
would earn their livelihood there to a particularly severe
physical environment.

The vertical tide range on the shores of small oceanic
islands is narrow. At Enewetak, it averages 0.8 m, and the
spring tide range is 1.2 m. Over the 18-year period 1952
to 1969, the highest recorded tide was +1.8 m and the
lowest was —0.1 m, relative to prior lowest low water.
Despite this small range, many events within the intertidal
region differ markedly from those occurring below datum
and supratidally. Few marine organisms can tolerate the
supratidal fringe, wetted only by rain and sea spray and
subject to intense heat for long periods. Here physical fac-
tors profoundly affect the nature of the substrate. Within
the intertidal zone, conditions are less severe but still
stringent; substrate temperatures commonly reach 38°C
(Wiebe, Johannes, and Webb, 1975). Dissolution of lime-
stone reef rock substrate by rain and biological de-
struction by boring organisms are important processes
(Tracey and Ladd, 1974). All intertidal habitats are subject
to strong insolation and rain at low tide, and windward
intertidal environments bear the brunt of heavy wave
action at high tide. At the reef rim, coralline algae grow
and accumulate calcium carbonate rapidly. Their growth
also cements detritus and rubble into reef rock (Smith and
Harrison, 1977; Tracey and Ladd, 1974).

This chapter is dedicated to the memory of the late
Paul J. Leviten.

This chapter summarizes the present state of
knowledge of the ecology of intertidal environments at
Enewetak. It emphasizes the windward, seaward reef plat-
forms for several reasons:

1. They have been the most thoroughly studied, both
geologically and at several trophic levels of the ecosystem.

2. They represent a habitat type of widespread
occurrence throughout the tropical Indo-West Pacific (IWP)
region on both oceanic and continental islands and on con-
tinental shores (Kohn, 1971); data from Enewetak thus
provide a basis for comparisons with other IWP areas.

3. They afford significant comparisons and contrasts
with adjacent subtidal coral reef habitats; in contrast to
such reefs they are physically harsh rather than equable,
and their topography is simple rather than complex (Kohn,
1971).

In their pioneering study of the Enewetak coral reef
community, Odum and Odum (1955) characterized the
basic pattern of six physiographic zones comprising the
windward interisland reef 0.4 km north of Japtan (Muti)
Island, from the seaward edge toward the lagoon:

1. Windward buttress zone. Spurand-groove or surge
channel-andbuttress zone, just seaward of the highest part
of the platform, the coral-algal ridge. This is the upper
portion of the inaccessible mare incognitum (Smith and
Harrison, 1977).

2. Coral-algal ridge. "A low, narrow ill-defined strip of
limestone about 50 feet (16 m) wide" (Odum and Odum,
1955). Soft, fleshy algae (such as Dicti/osphaeria
intermedia, Lobophora uariegata, Ceramium, Dict];ota, and
Caulerpa elongata) cover the irregular surface, and the
crustose coralline alga Porolithon occurs in small patches.
Small clumps of corals (Acropora palmerae, Pocillopora.
Millepora p/atyphyZ/aj occur in protected sites. Algal ridges
are more extensive on island than interisland reefs.

3. Encrusting zone. "The first 200 ft (66 m) down-
stream from the ridge is a high, gently sloping plateau that
at low spring tide is covered with only 6 in.
(115 cm) of water. It is relatively the smoothest area with
corals being either of a flat encrusting growth form or re-
stricted to low rounded 'heads' but little raised above the
general reef surface. The range between tops of heads and

139

140

KOHN

ridges and the bottoms of depressions is only about 1 ft
(30 cm). As on the coral-algal ridge zone, sheets of yellow
Acropora and Millepora are conspicuous. In addition there
arc scattered low, rounded heads of Porites lobata and sev-
eral species of faviids ...living coral colonies on these low
heads are often crescent or doughnut shaped probably
because the higher center portions are killed periodically
by exposure during exceptionally low spring tides. Filamen-
tous red, brown, green, and blue-green algae form heavy
encrusting mats over all of the zone which is not covered
by coral, there being no areas of white sand as in the back
reef zones. Small sea anemones are abundant, occurring in
clusters throughout the algal mat. ...Coral cover is much
less than half the surface area. ...The zone receives pulses
of foam-water as the breakers throw rolls of water up on
the plateau. Since there is a distinct slope the current is
always strong even at low spring tide when the water
pours steadily across like a broad mountain stream rippling
over a rocky bed" (Odum and Odum, 1955).

4. Zone of smaller heads, 5. Zone of larger heads, and
6. Zone of sand and shingle are the lagoonward zones dis-
tinguished by Odum and Odum (1955). They are com-
pletely subtidai and will not be further discussed here.

The following description of a windward reef platform
adjacent to an island is taken from Leviten and Kohn
(1980). The platform on the windward (east) side of
Enewetak Island (Fig. 1) is about 90 to 120 m wide, "and
mainly topographically simple or smooth, but certain por-
tions have numerous holes and depressions. Wave action is
extremely heavy and constant on the seaward bench mar-
gin at high tide, but is damped somewhat on the inner por-
tions by an extensive low coralline algal ridge en the sea-
ward margin." At Sta. F7 of Leviten and Kohn (1980),
"the inner 20 m is scalloped and pitted reef rock undergo-
ing chemical dissolution (Revelle and Emery, 1957). It
lacks macroscopic algal cover, but in certain areas bears a
thin, slippery film of blue-green algae [Calothrix
Crustacea]. ...An incipient algal turf begins —25 m from
shore and increases in luxuriance in a seaward
direction. ...A shallow swale, unique to this bench among
those studied, occurs between 50 and 65 m from shore.
This area is covered by several centimeters of water even
during low tide and harbors a healthy 2-cm-thick algal
turf. ...The bench is pitted and dissected between 65 and
80 m from shore. Algal cover is thick, and algal species
richness is higher than on other portions of the
bench. ...The Porolithon ridge is evident ~80 m from
shore and continues to the seaward margin of the bench
~100 m from shore, where it is dissected by numerous
small surge channels and grooves. The ridge has a scoured
aspect, possibly due to intense grazing activity by herbivo-
rous fishes at high tide, and lacks an extensive fleshy algal
cover, save for a film of blue-green algae."

In order to characterize the microhabitats of benthic
invertebrates more precisely, bench substrates are classi-
fied as shown in Table 1 (see also Leviten and Kohn,
1980).

;7V

^- *'^'-

•"**-■

'-?*.

ia,

r^rer-

■-a^S^

Fig. 1 The windward platform of Enewetak Island, a, look-
ing NE at a neap low tide. April, 1979; b, a 1-m^ quadrat on
the same area shown in a, indicating the smooth topography
of the substrate.

Temperatures recorded by Havens (1974) on intertidal
substratum exposed to air reach 36.5°C on exposed reef
rock on the windward reef platform, 38.5°C on bench
rock, and 39°C on rubble-covered beaches. The maximum
temperature recorded under rocks and in holes was only
32.5°C. Water temperatures in tidal pools reach 38°C, in
contrast to submerged reefs, which do not exceed 32°C.

GEOLOGICAL PERSPECTIVE

Shallow-water marine environments at Enewetak
extend back in time 50,000,000 years. A vertical borehole
drilled in 1952 (Ladd and Schlanger, 1960) reached vol-
canic bedrock after traversing 1300 m of calcareous
material derived solely from shallow reef-building and reef-
associated organisms. Fossils in the reef limestone just
above the discontinuity were of Eocene age, indicating a
long-term average su'^sioence rate of about 0.03 mm yr
(Menard, 1964). Reco.ery of land mollusc fossils from core
samples indicates that, during probably cooler parts of
the Neogcne, Enewetak stood higher above sea level than
at the present time (Ladd, 1958).

INTERTIDAL ECOLOGY

141

TABLE 1

Classification of Windward
Reef Platform Substrates

Sand

Extensive sand patches

Sand in depressions in reef limestone
Sand under coral rock
Coral rubble with or without sand

On flat bench surface

In depression on bench
Very thin layer of sand on bench

On flat bench surface

In depression on bench
Limestone bench bare of sand or algal turf

Smooth

Rough

In depression
Bare beachrock
Algal turf on reef limestone bench, typically binding sand

On smooth bench surface

On rough bench surface

In depression on bench surface
Dead coral boulder
Crustose coralline algae

The Quaternary record shows "at least four strati-
graphic intervals representing reef growth and associated
lagoonal sedimentation during relatively brief periods of
Quaternary interglacial high sea levels, overlying unconfor-
mities representing periods of emergence and weathering
during glacial lowering of sea level" (Tracey and Ladd,
1974). The uppermost unconformity, about 10 m below
present sea level, separates sedimentary rocks less than
6000 years old from sediments about 120,000 years old.
The limestone rock pavement constituting the present
extensive windward reef platform adjacent to Enewetak
Island (Fig. 2) is about 4000 years old and is a currently
developing unconformity (Tracey and Ladd, 1974; Bud-
demcier. Smith, and Kinzie, 1975). The latter authors
characterized the rock at and near the seaward algal ridge
crest as well lithified "poorly sorted coral rubble, coarse
sand, and obvious (?) coralline algae in a fine-grained cal-
careous matrix." The composition of this material suggests
that it was formed in a subtidal, sedimenting environment
very different from that at the site today. Geological and
radiocarbon studies of this material suggest the following
temporal scenario of the interplay of sea level changes and
biotic activities during the Holocene, leading to the inter-
tidal environments of Enewetak at the present cross sec-
tion of time: The sea was less than 3 m below its present
level 6000 years ago (ybp), rising to a maximum 3500 to
2000 ybp, a period of rapid reef growth. At 4000 ybp,
the age of the present surface rock when the corals shown
in Fig. 2 were living, sea level was probably about 0.3 m
higher than at present. From 2000 to 1000 ybp, emer-
gence and extensive surficial erosion of the reef accom-
panied a drop in sea level to its present level.

Fig. 2 Reef limestone surface in the inshore 20 m of the
windward platform at Enewetak Island, showing corals in
growth position and the effects of subaerial, marine, and
biogenic erosion. The present level of the rock is too high in
the intertidal zone for coral growth. Dark spots in small
higher pits and in the pothole at lower right are limpets.
Siphonaria normalis. The lens cap is 52 mm in diameter.

Shorter-term geochemical processes have been eluci-
dated in studies of calcification rates to be discussed
below. They show that the interisland windward reef plat-
form is accreting vertically at a rate of about 3 mm yr
(4 kg m~^ yr~^) in the absence of sea level change (Smith,
1973). If most of the products of calcification were
retained on the platform during the 1952 to 1970 period
in which there was no net sea level change, an estimated
shoaling of 5 cm would have occurred; if most net prod-
ucts of calcification were lost by transport to the lagoon,
no net change in platform level would have occurred.

Calcification rates subtidally on the windward reef
slope are somewhat lower, about 1 to 2 kg m yr in
the buttress or spur-and-groove zone, but 3 to 6 kg m
yr^^ where coral cover is virtually total at depths of 15 to
25 m (Smith and Harrison, 1977).

BIOLOGICAL PATTERNS AND
PROCESSES

Primary Productivity

The intertidal and shallow subtidal windward reef plat-
forms at Enewetak are highly productive, particularly
where covered by a dense algal turf. The mainly subtidal
interisland reef north of Muti Island, close to the site of the
original study by. Odum and Odum (1955) has been the
most intensively studied. Smith and Marsh (1973) mea-
sured primary productivity along two transects normal to
the seaward reef edge by two independent methods — rate
of oxygen production and rate of carbon dioxide
fixation — which gave results in close agreement. At Tran-
sect II (tr II; 340 m), the red crustose coralline alga
Porolithon onkodes and the brown alga Lobophora varie-

142

KOHN

gala were tha dominant cover organisnns of the algal ridge,
and corals were very sparse. For about 160 m lagoon-
ward, Porolithon and the turfy red coralline alga Jania
capillacea dominated; some zoanthids and holothurians
were also common. Over the next 180 m, the corals
Pontes. Acropora, and Heliopora increased in abundance
on the limestone pavement, and Porolithon continued.
Transect III (tr III; 270 m) was characterized by cover of
Porolithon at the algal ridge and Jania turf throughout.
Foraminifera and gastropod molluscs were common, but
there were virtually no corals on the transect. Water depth
over these transects at low tide was less than 1 m.

As Table 2 indicates, the transect with the greater
dominance of algae throughout (tr 111) was the more pro-
ductive. And as shown in Table 3, only the predominantly
algal turf areas were highly autotrophic; the other regions
produced about as much organic matter as they consumed.
Smith and Marsh (1973) also demonstrated that the C:02
metabolic quotient for the platform was very close to 1.0,
as prior authors had assumed but had not tested.

TABLE 2

Gross and Net Primary Organic

Productivity of an Interisland

Windward Reef Platform at Enewetak Atoll*

Daytime gross
productivity

Daytime net
productivity

Transect II

0.50
2690

0.25
1345

Transect III

0.97
4249

0.72
3154

Algal ridge

0.14
550

0 11
240

"Data from Smith and Marsh (1973) and Marsh
(1970). Upper figures: g C m~ h~'; lower figures:
gCm-2y-'

Primary productivity of algal ridge crest areas dom-
inated by Porolithon is much lower than the transect as a
whole (Tables 2 and 3). Marsh (1970) concluded that this
zone contributes much less to overall reef productivity than
the others and that the reef-building activities of the crus-
tose coralline algae are more important than their energy
fixing.

Bakus (1967) estimated net primary productivity of the
inner portion of the Enewetak Island platform where the
blue-green algae Calothrix Crustacea and Schizothrix cal-
cicola are the dominant plants. Because his method mea-
sured increased standing crop in cages that excluded large
herbivorous fishes but not small fishes or benthic inver-
tebrates, the result, about 440 g C m~ yr~', is undoubt-
edly an underestimate.

Nutrient Cycling

Because nitrogen in forms assimilable by photosyn-
thetic organisms is an important limiting factor of primary
productivity in the sea, the recent discovery that nitrogen
fixation occurs at high levels on Enewetak's windward reef
platforms (Webb and Wiebe, 1975; Webb, DuPaul, Wiebe,
Sottile, and Johannes, 1975) is an important contribution
to understanding coral reef-associated ecosystems. Studies
at tr II and tr III described above indicated increasing con-
centrations of nitrate and ammonium as water crosses an
island reef platform. The highest rates of production (about
1.5 nmoles cm~ h~' each of NO3 and NH4 ) occurred
on rock surfaces supporting a mixed algal turf dominated
by Calothrix Crustacea (about 80%) and Schizothrix cal-
cicola (about 20%). This rate of nitrogen fixation is com-
parable to those in managed agriculture (Wiebe, Johannes,
and Webb, 1975). Calothrix covered the surface as a thin,
yellow-brown film over large areas of the platform and
penetrated the limestone to a depth of several millimeters
(Webb and Wiebe, 1975). Although the nitrogen fixing,
chemoautotrophic bacterium Mtrobacter agilis colonized
slides placed on the substratum and after 4 weeks attained
densities high enough to fix NO3" at the observed rates

TABLE 3

Ratio of Productivity and Respiration on an Interisland
Windward Reef Platform at Enewetak Atoll

Habitat type

Daytime net*
productivity

Nigiittime*
respiration

Daytime gross*
productivity

24-hour
gross P:R

Reference

Coral-algal

0.4

0.8

1.0

Odum and Odum,
1955

Coral-algal (Tr II)

0.25

0.5

Smith and Marsh,
1973

Algal (Tr III)

0.72

025

0.97

1.9

Smith and Marsh,
1973

Algal ridge

0.11

0.04

0.14

1.8

Marsh, 1970

*g C m ^ h

INTERTIDAL ECOLOGY

143

(Webb and Wiebe, 1975), Wiebe, Johannes, and Webb
(1975) concluded that Calothrix is the most abundant and
important nitrogen-fixing organism. It is extremely tolerant
of the high range of temperatures and salinities characteriz-
ing its environment. Under experimental conditions, rates
of N fixation doubled between 27°C and 36°C; the limit-
ing temperatures were 24°C and 39°C. Experimental salin-
ities between 3% and 45% did not affect fixation rates
(Wiebe, Johannes, and Webb, 1975).

Predominantly algal reef flats are thus extremely impor-
tant as a source of fixed nitrogen for adjacent communities
and hence a critical source of their high productivity.
Wiebe, Johannes, and Webb (1975) concluded that fixed
nitrogen enters the rest of the reef ecosystem via three
routes: (1) herbivorous parrot fishes and surgeonfishes
that graze on Calothrix Crustacea have low assimilation effi-
ciency, and much of the organic matter in their food is
liberated as feces; (2) Calothrix growing in the surf zone is
broken off the substrate and moved downstream by
lagoonward currents, where it is more subject to herbivory;
"benthic algal fragments constituted by far the largest por-
tion of the net plankton on the windward interisland reef at
Enewetak, and Calothrix constituted 20 to 60% (by
volume) of these fragments" (Wiebe, Johannes, and Webb,
1975); (3) Calothrix may release much of its fixed nitrogen
into solution; in culture as much as 40 to 60% was
released as peptides and amino acids.

Concentration of reactive and organic phosphorus does
not change as water passes over tr II; at tr III,
reactive P decreased and organic P increased in concentra-
tion, both slightly but consistently (Pilson and Betzer,
1973). Samples of reef rock dominated by Schizothrix
actively take up phosphorus at a mean rate of 0.27 nmole
P cm h~' during the day but also continually lose
labeled P; mats dominated by Jania showed very little net
uptake or loss (Pomeroy, Pilson, and Wiebe, 1974). These
authors did not detect any special mechanism for retention
of phosphorus by the windward reef platform community.
Phosphorus incorporated by Schizothrix may be cycled
through the food web by herbivorous fishes and inver-
tebrates, which excrete it as phosphate.

Calcium Transport and Calcification

In addition to monitoring organic carbon production.
Smith (1973) was able to use the CO2 system of the
interisland windward reef platform to determine gross cal-
cification rates. Both a transect dominated by algae (tr II)
and one with both algae and corals (tr III) added CaC03 at
a rate of about 4 kg m^ yr \ comparable to similar
habitats elsewhere (Smith and Kinsey, 1976). Smith (1973)
estimated the erosion rate at less than 1 kg m~^ yr~\ suf-
ficiently close to the standard error of the calcification rate
to be ignored. Net calcification on the interisland reef plat-
form thus probably approximates gross calcification.
Locally production may be much more rapid: coralline
algal pavement at the reef rim produces CaCOs at a rate
of 8 to 16 kg m"^ yr"MSmith and Harrison, 1977).

Bacteria

DiSalvo (1973) noted the occurrence of a re-
duced layer of sand, suggesting bacterial decay of plant
material, at the foot of beaches on the windward sides of
Enewetak and Parry Islands.

Benthic Flora

Studies of the algae of the windward platforms subse-
quent to the initial work of Odum and Odum (1955) have
noted some differences but have been neither thorough
nor frequent enough to determine whether they indicate
spatial or temporal patchiness, or both, or long-term
trends. At the algal ridge crest and for about 200 m
lagoonward on interisland platforms, the crustose
Porolithon on/codes and the turf-forming Jania capillacea
continue to dominate the surface of the lithified reef rock
described above (Buddemeier, Smith, and Kinzie, 1975).
Bailey-Brock, White, and Ward (1980) characterized four
macroalgal zones inshore of the algal ridge crest on the
island reef platform at Enewetak Island (Table 4) where
turf-forming algae dominate. The algal turf is much
thicker — about eight times as much biomass — and holds
much more water at low tide in their zones 3 and 4 than
in the inshore zones. Such turfs are the characteristic algal
growth form in physically stressed tropical environments.
They arc more resistant to desiccation at low tide and to
herbivory than separate individual plants, which are more
productive and better competitors but are less resistant to
harsh physical conditions (Hay, 1981).

The importance of the widespread blue-green alga
Calothrix Crustacea has been mentioned above. This
diaphanous, yellow-brown film covers large areas of the
windward flat. "Along the upper intertidal bench zone
another growth form of the same species occurs as a
black, feltlike mat up to 5 mm thick. At low tide, most of
this mat dries out. It is not heavily grazed by fish owing to
the shallowness of the water in which it grows. In areas of
the windward reef flat, dominated by other algae, C. Crus-
tacea is found ubiquitously as an epiphyte" (Wiebe,
Johannes, and Webb, 1975).

In their initial study, Odum and Odum (1955) dis-
tinguished several ecological groups of primary producers
in the intertidal region of the interisland transect: phyto-
plankton; zooxanthellae in coral F)olyps, sea anemones, and
Tridacna; filamentous algae within skeletons of living
corals; encrusting filamentous, crustose coralline, and
fleshy green algae affixed to smooth and rough surfaces;
and algae associated with dead coral heads. They
estimated, admittedly crudely, dry biomass of primary pro-
ducers. In the coral-algal ridge zone, algae in corals and
encrusting and free-living algae contributed about equally
to the total estimate of 635 g m~^. In the encrusting
zone, encrusting algae and algae boring within the rock
substrate accounted for most of the biomass. The esti-
mates of Bailey-Brock, White, and Ward (1980) for com-
parable zones on an island platform are similar; the latter
researchers reported order-of-magnitude lower biomass

144

KOHN

TABLE 4

Distribution and Biomass of Algae Across the Windward
Reef Platform at Enewetak Islet'

Zone

Crest

Distance from shore (m)

60-71

40-60

-40

0-20

Predominant algal species

Porohthon spp.

Jania sp.

Dictyosphaeria cauernosa

Padina japonica

Cauterpa spp

Valonia spp

Boodlea composita

Gelidiella sp.

Poltjsiphonia sp.

Chdophora hemispbaerica

Codium edule

Total percent cover

80-95

30-

-40

10-15

Mean dry wt (g m~^) platform

850

625

'Data from Bailey-Brock, White, and Ward, 1980. Figures in body of table
are percent cover.

flncludes Acetabularia clauata.
:t:lncludes Ceramium sp.

inshore (Table 4). The coralline Jania capillacea and the
brown Sphacelaria sp. are predominant algae in the
encrusting zone (Miller, 1983).

Benthic Fauna: Abundance and
Distribution

Protozoa

Foraminifera containing symbiotic algae are the largest
and most prominent benthic intertidal Protozoa at
Enewetak. They are probably restricted to cracks and
holes in the reef rock that afford both adequate light for
photosynthesis and shelter from grazing fishes and inver-
tebrates (Lipps and Delaca, 1980). These authors reported
the presence of several genera at Enewetak but did not
indicate which were intertidal. Hirshfield, Charmatz, and
Helson (1968) noted that the family Miliolidae comprised
82% of the Foraminifera on the Parry (Medren) Island;
seven rarer taxa were also present. Foraminifera containing
zooxanthcllae are often important contributors to both cal-
cification and organic carbon fixation in coral reef associ-
ated communities (Smith, 1977). Benthic Foraminifera are
sufficiently abundant occasionally to be important food
items of some xanthid crabs (Havens, 1974) and fishes
(Hiatt and Strasburg, 1960).

Porifera

Clionid sponges that excavate chambers in the herma-
typic coral Pontes lutea on interisland platforms are ecolog-

ically the most important Porifera of the Enewetak inter-
tidal and shallow subtidal zones. The dominant species are
Aka sp. cf. A. diagonoxea and Cliona sp. cf. C. quadrata
(Highsmith, 1981). They initiate burrows as cylindrical
excavations about 200 fim in diameter and 300 nm deep.
The burrow is then extended 2 to 3 mm, after which it is
expanded into a chamber 5 mm or more in diameter.
Chambers of this size are found within 3 months of expo-
sure of dead coral skeletal surface, especially on the under-
sides of P. lutea microatolls. Highsmith (1981) showed that
sponges are the most common infaunal associate (in 86%
of coral heads examined) and the most important bio-
eroders of corals at Enewetak. The overall effect of this
bioerosion is not known. Smith (1973) suggested that the
rate of CaC03 removal by sponges is probably something
less than one-fourth the rate of calcification.

Cnidaria

The predominant cnidarian of low intertidal ( + 0.15 to
0.3 m; Havens, 1974) interisland platforms is the herma-
typic coral Porifes lutea. Here its typical growth form is
the microatoll. As described by Highsmith (1980), "the
tops of these massive corals are killed by prolonged expo-
sure during seasonally low tides. The coral tissue around
the periphery of the head survives and continues to grow
radially outward resulting in the characteristic microatoll
form." Pontes lutea microatolls tend to become detached
from their substrate because (1) their inability to maintain
live tissue under shaded portions of the jseriphery results

INTERTIDAL ECOLOGY

145

in an overhang of tissue-covered, growing skeleton, and (2)
they are weakened by the skeletal boring activity of clionid
sponges and sea urchins. Detached, living pieces of
P lutea microatolls tend to move lagoonward across the
interisland flat in the unidirectional water flow. They con-
solidate in a zone along the lagoon edge, where they con-
tribute importantly to active reef growth (Highsmith,
1980). Odum and Odum (1955) noted the presence of less
abundant corals; these are listed above in the description
of the coral-algal ridge taken from their paper. They
referred the anemones they noted to the genus
Actiniogeton. Miller (1983) noted a common, undetermined
high and mid-intertidal anemone.

The most abundant intertidal hydroid is the inconspicu-
ous tropical and subtropical cosmopolitan species
Di^namena chsioides. Its irregular branching stems extend
5 to 15 cm from a thick hydrorhiza, but it is often
obscured by heavy encrustation of detritus and algae. It
occurs intertidally in beachrock crevices at the north end
of Enewetak Island and commonly on the outer portion of
the island reef platform (Cooke, 1975).

Annelida

As on reef platforms elsewhere dominated by algal
turfs (Kohn and Lloyd, 1973; Kohn and White, 1977),

polychaete annelids arc a numerically dominant component
of the benthic invertebrate community at Enewetak. Blocks
of reef limestone chiseled from the central portion of the
windward Enewetak Island platform, including the overly-
ing algal turf, support a mean density of about 90,000
polychaetes m"^ (range 82,000 to 100,000; 27 to 39
species; biomass 7.4 to 9.3 g m~ dry weight). About
10% of the polychaetes (mean 8000; range 400 to 32,700
m~^; 2 to 34 species) were associated with the algal turf
alone, indicated by samples of turf scraped from the reef
rock surface (Bailey-Brock, White, and Ward, 1980).

Species richness, population density, and biomass of
polychaetes increased from shore toward seaward edge,
with maxima at 56 to 66 m, closely paralleling the
biomass of algal turf (Table 4, Fig. 3). Nearshore
tidepools that hold more water than the surrounding plat-
form support higher densities of polychaetes but not
greater algal biomass. Near the coralline-dominated algal
ridge (85 m from shore; Table 4), limestone blocks con-
tained an estimated 60,000 polychaetes of 25 species
m"2 (Bailey-Brock, White, and Ward, 1980).

Also typical of similar habitats elsewhere (Kohn and
Lloyd, 1973; Kohn and White, 1977), the family Syllidae
are both the most diverse (31 species) and the most abun-
dant (up to 48,500 m~ ) polychaete family on the
Enewetak windward platform (Bailey-Brock, White, and

20 30 40 50 60
Distance from Shore, m

Fig. 3 Algal biomass and species richness, abundance, and biomass of polychaetes along a
shoreward-to-seaward transect on the windward island platform at Enewetak Island. From Bailey-
Brock, White, and Ward (1982).

146

KOHN

Ward, 1980)^ However, most syllids are among the small-
est polychaetes present. Their contribution to biomass is
small in proportion to their numbers; however, their pro-
ductivity is unknown and may well be very high.
Tubicolous members of the families Spionidae, Sabellidae,
Capitellidae, and Dodecaceha (family Cirratulidae), and the
errant Syllidae predominated in the samples within 30 m
of shore where algae are sparse. In the central portion of
the platform with algal turf, the latter four taxa, Chaetop-
teridae, and the errant Amphinomidae are the most abun-
dant polychaetes. The thick layers of smooth, encrusting
coralline algae of the ridge support primarily Nereidae, Syl-
lidae, Vermi/iopsis (Serpulidae), Spionidae, and Eunicidae.

Polychaetes are also the most common invertebrates
associated with Porites lutea heads on the interisland plat-
form (Highsmith, 1981: App. II, Part C, Nos. CI, C3, C4,
C5, C6, C9, 39, 41, 84, 87). Syllids dominated numeri-
cally (121 individuals), followed by eunicids (67, but with
the greatest biomass), and cirratulids (60) in a sample total-
ling 264 polychaetes from nine heads. Highsmith (1981)
considered most of the polychaetes to be nestlers, mainly
in empty chambers that had been excavated by sponges in
the coral skeleton.

Sipuncula

The only information on the distribution of inter-
tidal sipunculans at Enewetak appears to be Highsmith's
(1981: App. II, Part C) report of limestone-boring species
in the skeletons of Porites lutea microatolls on the inter-
island platform. Seven corals examined contained 58 sipun-
culans of at least seven species. Aspidosipfion muelleri
comprised 60% of the sample. Other identified species
were Cloeosiphon aspergillus. Lithacrosiphon gurjanovae,
and Paraspidosiphon gigas. Sipunculans probably occur
densely in the reef rock substrate of the platform; as noted
below they are the sole food of a common predatory gas-
tropod, Mitra Htterata.

Echinodermata

The sea cucumber Hohthuria atra is the most conspicu-
ous invertebrate of the interisland windward reef platforms.
Bakus (1973) estimated 0.1 m~ north of Enewetak
Island. Maximal densities of from 3 m~ (Webb, DuPaul,
and D'Elia, 1977) to 5 to 35 m"^ (Bakus, 1973) occur in
areas protected from high water velocities and surf. The
highest densities occur in depressions and gutters that
retain water at low tide (Ebert, 1978).

The sea urchins Echinometra mathaei and
Echinostrephus aciculatus are commonly associated with
Porites lutea microatolls and dead coral heads. They
shelter adjacent to these and weaken them by eroding
skeletal material, presumably by mechanical action of both
teeth and spines. The densest populations of these echi-
noids also occur on slightly subtidal portions of the
Enewetak windward reef platform. Here densities of
Echinometra mathaei reach 6.5 m~ and Echinostrephus

aciculatus, 1.1 m~ ; the two species have similar environ-
mental requirements and are significantly positively associ-
ated (Russo, 1980). Although probably second in impor-
tance to sponges as bioeroders, the roles of these urchins
in removing CaCOa has been estimated more quantita-
tively. Russo (1980) calculated erosion rate by the two
species together as 325 g m^ yr~^ on the mid-portion of
the reef platform and at 108 g m~^ yr~' on the outer
platform, where urchin densities are lower. This represents
removal of about 2 to 8% of annual CaCOs deposition
(Smith, 1973).

At the seaward edge of some windward platforms,
especially on Japtan Island, Heterocentrotus trigonarius
occurs commonly, wedged in cavities on the outer portion
of the coral-algal ridge. It is the only sea urchin species in
this surf-swept zone, and its massive body wall and thick,
heavy spines adapt it to this harsh environment. In addi-
tion, H trigonarius appears to have a physiological require-
ment for considerable water movement (Ebert, 1982).

Ebert (1982) also estimated growth and mortality rates
of Heterocentrotus trigonarius and Echinometra mathaei at
Enewetak. The former is a long-lived species (probability
of annual survival is 0.97) that grows slowly and has a low
instantaneous annual mortality rate per individual (0.006).
Echinometra mathaei grows an order of magnitude more
rapidly but has a shorter life span (probability of annual
survival is 0.42) and a higher and quite variable mortality
rate (1.26 in 1978; 0.48 in 1979).

Mollusca

Small prosobranch gastropods are particularly charac-
teristic benthic invertebrates of windward reef platforms.
Detritus-feeding vermetids, mainly Dendropoma psaro-
cephala. occur at densities of 150 to 1100 m^ (Miller,
1983). Detritus-feeding and herbivorous members of the
family Cerithiidae attain densities of 800 m~ ; Cerithium
alveolus is the most common species (Miller, personal com-
munication). The cowry C\;praea moneta occurs centrally
on the reef platform, typically in male-female pairs, at den-
sities of 0.2 to 0.7 m~ and is more abundant in tidal
pools on the platform (2.0 to 6.7 m^ ) and subtidally in
the quarry (Renaud, 1976). Carnivorous neogastropods,
represented most abundantly by the families Conidae and
Muricidae and secondarily by the Mitridae, Buccinidae, and
Vasidae, have been more intensively studied. The first two
families represent about 50% and 40%, respectively, of the
predatory gastropods present (Kohn and Leviten, 1976;
Kohn, 1980). Substrate topography is the most important
factor controlling population density and species diversity
of these gastropods. Depressions in the bench surface that
collect coral rubble and flatter areas with thick algal turf
that binds sand provide refuge from the harsh physical
stresses of desiccation and rain at low tide and heavy wave
action at high tide, and probably from prcdation (Ayal and
Safriel, 1982).

Population densities of Conus species and of other
predatory gastropods are significantly higher on portions of

INTERTIDAL ECOLOGY

147

bench with algal turf or natural or artificial depressions
than on adjacent smoother, barer bench of the type shown
in Fig. lb Species richness of Conus and of total preda-
tory gastropods is also significantly lower in the last
microhabitat type (Table 5; Kohn and Leviten, 1976;
Leviten and Kohn, 1980). The most common species of
Conus are C. ebraeus. C. sponsalis, C. chaldaeus, and
C. miliaris. Rarer species are C. frigidus. C. flavidus.

C. catus. C. rattus. and C. retifer; the last two did not
occur in the quantitative samples summarized in Table 4.
The predominant muricids are Morula granulata. M. uva.
Drupa morum, and D. ricina; less common are

D. arachnoides. Maculotriton serriale, and Thais fuberosa.
Drupa morum and D. ricina are as abundant on smooth,
bare bench as they are where refuges are present. Their
depressed, limpet-like shells and disc-shaped feet permit
more tenacious adhesion to the substrate than the longer,
narrower feet of most other gastropods present. Vasum
turbinellum (family Vasidae) is the most common other gas-
tropod present, followed by the mitrids Mitra litterata.
Mitra cucumerina. Vexillum cancellarioides, and Imbricaria
punctata (Kohn and Leviten, 1976).

Co-occurring species of Conus do not use different
types of microhabitat (listed in Table 1) differentially. All
species observed are typically (96% of individuals) inactive
in refuges during daytime low tides, when physical stresses
are probably harshest. Activity increases abruptly to a high
level in all species in late afternoon and early evening,
unless water flow is too strong (50 cm s~'), as the animals
move over the smooth bench surface and feed during the
night. They do not home to the same site used the previ-
ous day, and we detected no evidence for competition for
protected sites. These results led to the conclusion that the
Conus species on the windward island platform at
Enewetak partition neither microhabitat nor temporal
resources (Leviten and Kohn, 1980).

A few nudibranch gastropods occur in protected sites
on windward platforms. Chromodohs geometrica. Okadaia
elegans, and a few other members of the superfamily Dori-
dacea occur under rocks intertidally, and the aeolidiacean
Herviella mietta occurs in tide pools (Young, 1967).
Characteristic of inner zones of the windward platform is
the abundant pulmonate limpet Siphonaria normalis, which
occurs at population densities of 400 m~ on flat substrate

TABLE 5

Abundance and Diversity of Predatory Gastropods in Different Microhabitats
on Windward Reef Platforms at Enewetak*

All

predatory

Area

Conidae

Muricidae

gastropods

sampled.

No. of

Density

No. of

Density

No. of

Density

Microhabitat type and census areas

species

Ncm"^

species

No. m"*

species

No.m-*

Smooth, bare portions without refuges

106

0.7

1.1

1.8

(A,. As, A3)

Smooth portions with algal turf

2.5

4.4

6.9

binding sand (B)

Smooth, bare portions with natural

118

3.3

0.9

4.6

refuges (C,, C3, Q, Cg)

Smooth, bare portions with artificial

4.1

2.9

7.5

refuges (C2)

Totals (sample sizes in parentheses)

301

(688)

(526)

(1297)

"Data from Kohn and Leviten, 1976.

The different Conus species arc typically zoned across
island platforms, with the peak abundances of C. ebraeus
closest to shore, C. chaldaeus and C. coronatus intermedi-
ate, and C sponsalis closest to the outer edge. However,
distributions vary at different study sites and at different
times at the same site (Leviten and Kohn, 1980). In all
four of these species, shell length decreases significantly
with distance from shore, but in C. ebraeus and possibly
C. sponsalis it increases again near the outer limits of their
distribution.

that dries at low tide and to 3500 m~ in shallow tide
pools. Its body size and population distribution are deter-
mined by the foraging behavior of its predator, the proso-
branch gastropod Thais armigera (Menge, 1973).

Siphonaria normalis also occurs commonly on a
smooth, sloping beachrock shore on the lagoon side of
Enewetak Island at the +0.6- to +0.9-m level. It becomes
active when just covered by a rising tide or just uncovered
by a falling tide, moving from its home scar in an
unpredictable direction to forage on microalgae (Cook and

148

KOHN

Cook, 1981). After foraging, S. normalis returns to its
home scar over its outbound route (Cook and Cook,
1978) Activity ceases for long periods of emersion during
neap tide periods. Cook and Cook (1981) found no rela-
tion between distance moved and either body size or graz-
ing interval in S. normalis.

In general, gastropods inhabiting the windward reef
platforms are small in comparison with congeners occupy-
ing subtidal reef habitats. Few Conus exceed 25 mm in
shell length in the former habitats (Kohn, 1971, 1980),
and extensive field observations suggest the same to be
true for other gastropod taxa.

Arthropoda

Xanthid crabs are the most prominent intertidal Crusta-
cea at Encwetak. Havens (1974) studied their distribution
and comparative ecology and presented detailed informa-
tion on their habitats and habits. He listed the most
abundant species by zones as follows. (Population densities
are given in parentheses.)

Windward Buttress Zone and Coral-Algal Ridge.

Paraxanthias notatus most common, then Liocarpilodes
integerhmus and Circloxanthops cauatus. associated with
both dead coral and coralline algae; Globopilumnus glo-
bosus and Dacr[;opilumnus emerita in algal rock on the
highest parts of the algal ridge; Chlorodiella laevissima on
dead coral, and Trapezia and Tetralia spp. in living corals.
Crabs other than xanthids in this zone include species of
Pachygrapsus, Percnon, and Plagusia (Grapsidae) and
Thahmita (Portunidae).

Inner Portion of Algal Ridge. Eriphia scabricuh on coral-
line algal mounds; this species, Dacri/opilumnus rathbunae
(14 m~^), Liocarpilodes biunguis and L. pumilis (5 m^^) in
the rims of rubble-filled former surge channels; L^dia annu-
lipes (10 m^ ) on inner algal mounds.

Smooth Reef Platform. Liocarpilodes biunguis (33 to
104 m~^; this is the most common intertidal xanthid in
the Marshall Islands), Xanthias lamarcki. Etisus bifrontalis,
and Pilodius areolatus in broad, shallow tidal pools such as
the swale at our Sta. F7 described above (Leviten and
Kohn, 1980: Fig. 1). The most diverse crab fauna of all
intertidal environments Havens studied at Enewetak occurs
here, including representatives of the families Atelecyclidae
(Kraussiaj, Portunidae (Thalamita. Portunus), Grapsidae
(Pach^igrapsus, Percnon), Ocypodidae (Macrophthalmus),
Majidae (Micippa), and Parthenopidae. On the northern
part of the Enewetak Island windward platform the algal
turf is reduced to a film of blue-green algae, possibly
because of grazing by herbivorous fishes at high tide as
noted earlier. Here Liocarpi/odes biunguis and Leptodius
davaoensis (29 m~^) are the most common xanthids, and
Leptodius sanguineus, Eriphia scabricula (9 m ), and
E. sebana also occur. At night, the last species is the most
prominent large crustacean. The grapsid species mentioned
above also occur here. The xanthid fauna of the innermost
part of the platform is restricted to small Liocarpilodes

biunguis and Pachi/grapsus minutus that occur uncommonly
in small holes.

Scalloped and Pitted Reef Rock. The dominant xanthid
here is L^/dia annulipes (10 m"^), found in holes and crev-
ices.

Beachrock. Ridges of exposed, intertidal beachrock occur
adjacent to both seaward and lagoonward sides of islets
(Kohn, 1981: Fig. 1) at Enewetak. The predominant
xanthid crabs in seaward beachrock areas are Pseudozius
catjstrus. Lydia annulipes, and Liocarpilodes biunguis. Holes
and cracks in intertidal beachrock on the lagoon side are
an important habitat of Eriphia scabricula (15 m ). Eriphia
sebana and L annulipes (2 m~ ) also occur here.

Rubble-Covered Beaches. Loose coral rubble often covers
sand and gravel of beaches at Enewetak; this is particularly
apparent on the windward side after storms (Kohn, 1980:
Fig. la). This habitat supports a characteristic crab fauna
of the xanthids Pseudozius caysfrus. Leptodius davaoensis.
L. gracilis, and L. sanguineus (2 m~ ) and the grapsid
Pach\^grapsus planifrons. Leptodius davaoensis is more
common than L. gracilis in the low intertidal (16 vs. 4
m ), but the latter tends to replace the former in the high
intertidal (15 vs. 5 m~ ). Pseudozius capstrus attains a
density of 21 m~ even higher on the beach. Species of
Pseudograpsus and Ci^clograpsus occur higher, on sand
under rocks at the supra tidal fringe.

Porites Microatolls. The highest diversity of Xanthidae at
Enewetak occurs on interisland reef platforms in
association with the Porites microatolls discussed previ-
ously Among the 35 species of xanthids are Pilodius areo-
latus. P pilumnoides. Xanthias lamarcki, Liomera bella,
L. pallida. Chlorodiella cytherea. C. laevissima. Para-
medaeus simplex, Medaeus elegans. Etisus bifrontalis. E.
demani. Lachnopodus subacutus, and Carpilius convexus.
and the portunid Thalamita picta. Highsmith (1981: App.
II, Part C) noted a somewhat different set of crabs in this
habitat. He also reported acrothoracican barnacles in the
coral heads.

Havens (1974) considered the five species listed in
Table 6 as the characteristic low intertidal xanthids of
Enewetak. As the Table indicates, they tend to partition
microhabitat resources. Table 6 also lists the three charac-
teristic high intertidal species; the vertical distributions of
the two sets of species do not overlap in the localities
studied. These species also occupy quite distinct micro-
habitats from each other. Because Eriphia scabricula and
Lydia annulipes occur at the same localities and occupy
similar microhabitats but at different heights, they are
good indicators of tidal level.

A large crustacean not seen on windward reef plat-
forms during the day but active there at night is the
macruran Panulirus penicillatus During the day, it remains
in subtidal dens on the reef front. Its major peak of activity
follows dusk, when it crosses the reef crest to forage; a
minor peak occurs at dawn. At Enewetak Island, it was
observed to use primarily depressions covered with a thin

INTERTIDAL ECOLOGY

149

TABLE 6

Vertical Zonation and Microhabitats of the Eight
Dominant Intertida! Xanthid Crabs at Enewetak*

Species

Primary microhabitat

Low Intertidal Species (Approximately +0.3 to 0.9 m)

Liocarpilodes biunguis
Eriphia scabricula

Leptodius datxioensis

Leptodius sanguineus

Dacr\;opilumnus
rathbunae

Reef flats, in small holesf
Reef flats without algal turf

binding sand, in large

holest
Reef flats witfi tfiick algal turf;

under small rocks^:; in

large holes
Reef flats and rubble-covered beaches;

under large rocks;): and overhangs;

overhangs in large holes
Eroding algal rock, beachrock,

reef blocks with thin algal film;

in sipunculan burrows

(small holes)

High Intertidal Species (Approximately +0.8 to +1.3 m)

Pseudozius cavstrus

Lydia annulipes
Leptodius gracilis

Reef flats and rubble-covered
beaches; under large
rocks mainly on gravel; less
common on sand under rocks;

Eroding reef rock, beachrock;
in large holes

Reef flats and rubble-covered
beaches; on sand or
gravelly sand under rocks of all sizes

•Data from Havens, 1974

fSmall holes have aperture si7e (height + width) <17 mm;
large holes have aperture size >17 mm.

4:Small rocks are <23 cm long; large rocks are >23 cm long,

layer of algae; at Ananij it occurred most commonly on a
reef with Pontes lutea. Acropora spp., and mats of the
brown alga Turbinaria; at Enjebi it occurred in larger
expanses of coral, primarily P lutea and some Acropora
spp. (McCollum, 1981).

Of the six species of shallow water stomatopods at
Enewetak, Conodact\^lus incipiens is the most abundant,
uses the widest range of habitats, and occurs most com-
monly in the intertidal zone, it Is often seen in isolated
shallow tidepools. Smaller individuals predominate in the
higher, inshore portion of the windward platforms. The
other species are mainly subtidal, but Haptosquilla gli^pto-
cercus occupies coral rubble and holes constructed by
other invertebrates on the windward platform, and it is
active in tide pools. Gonodact\^lus platiisoma is associated
with the bases of microatolls on the intertidal platform
northeast of Enewetak Island, and G. micronesica and
juveniles of G smithii occur in rubble in the same region
(Reaka and Manning, Volume II, Chapter 17, this
publication).

Of the few smaller intertidal crustaceans known from
Enewetak, the amphipod Melita celericuh occurs on the

undersides of rocks from mid-intertidal to subtidal. In a
transect on the lagoon side of the north end of Enewetak
Island it reached densities of 104 m~^ at tidal levels of
0.2 to 0.3 m (Croker, 1971). In this habitat, it was associ-
ated with other species of amphipods as well as with iso-
pods and tanaids.

Hermit crabs, primarily Clibanarius corallinus (54% of
total censused) and Calcinus laeuimanus (38%), occur on
the Enewetak windward platform at densities of 3 to
65 m (Miller, personal communication).

Benthic Fauna: Trophic Roles and Interactions

Although the standing crop, number, and biomass of
several intertidal benthic invertebrate taxa have been
assessed at Enewetak, very few quantitative data exist on
their rates of resource utilization or their population
dynamics. This section is thus mainly limited to summariz-
ing available information on the roles of the numerically or
biomass-dominant taxa, and others that have Been studied,
in the trophic structure of their community.

Suspension and Deposit Feeders
on Small Particles

Tube-dwelling polychaetes predominate numerically in
this category, especially where depressions retain water at
low tide near shore on island platforms. Here the family
Spionidae, chiefly Pseudopolydora antennata and Microspio
microcera, averages 4300 individuals m ^. These animals
are selective detritivores. With a pair of long, tentacIe-Iike
palps they catch food particles both in susp)ension and
deposited on the substrate. The numerically dominant
suspension feeder is an unidentified filter-feeding sabellid
polychaete (5900 m"^). Next in abundance is the deposit-
feeding capitellid polychaete Leiochrides sp. In the
encrusting zone, common polychaetes in this category are
the chaetopterid Phi;llochaetopterus ramosus and the cirra-
tulid Cirriformia semicincta (Bailey-Brock, White, and
Ward, 1980).

Even less conspicuous than the polychaetes but
bionomically important are sponges oi the genera Cliona
and Aka, the most important boring organisms of coral
skeletons at Enewetak (Highsmith, 1980, 1981); they also
penetrate reef limestone. Tropical sponges effectively
remove a high proportion of particles in the T^m range
from the water they pump through their bodies, and unlike
other suspension feeders they are able to subsist
exclusively on bacteria and smaller particles (Reiswig,
1971).

The most conspicuous deposit-feeding invertebrate on
interisland platforms is Holothuria atra. From analysis of its
gut contents, fecal pellets, and the surrounding sediment,
Webb, DuPaul, and D'Elia (1977) showed this sea
cucumber to feed selectively on materials considerably
richer in organic content than the adjacent sediment, and
they calculated its feeding efficiency at 40%. A median-
sized H. atra (60 g) passes about 80 g (dry weight) of sedi-

150

KOHN

m&nt a day. Webb et al. (1977) estimated that they dis-
solve about 1% of the ingested CaC03, equal to 2 5 g m^
d~', or to about 25% of the net calcification rate on the
reef flat.

Planktivores

(1955) estimated dry biomass of
^ in the coral-algal zone and of

Odum and Odum
corals at about 100 g m "^ in the coral-alga
corals and anemones in the encrusting zone at about 50 g.
They classified them as herbivores because of their utiliza-
tion of zooxanthellae, but the predominant intertidal coral,
Pontes lutea (Highsmith, 1980, 1981), probably also feeds
on zooplankton at night.

Inshore, the hydroid Di/namena crisioides is pre-
sumably a planktivore.

Herbivores

Herbivory by invertebrates on the windward reef plat-
forms has been most thoroughly studied in crabs of the

family Xanthidae. Of the eight common species studied by
Havens (1974), seven are primarily herbivorous. In these,
78% to 100% of all individuals examined contained algal
food in their stomachs. Table 7 summarizes their feeding
habits and food based on Havens' findings. Predominantly
herbivorous nereid and eunicid polychaetes (author's obser-
vations) also occur in the outer zones, but they are much
less abundant than particle-feeding polychaetes (Bailey-
Brock, White, and Ward, 1980). Centrally on the reef plat-
form, the cowry C\;praea moneta eats primarily Jania capil-
lacea, to which it is preferentially attracted by distance
chemoreception (Rcnaud, 1976). Inshore, small but
numerous herbivores include the pulmonale limpjet
Siphonaha normalis, a grazer on microscopic algae (Menge,
1973; Cook and Cook, 1978, 1981), and cerithiid
prosobranch gastropods.

Odum and Odum (1955) estimated herbivore biomass
at about 23% and 8% of plant biomass on the coral-algal
ridge and encrusting zones, respectively, of the interisland
reef. They considered corals and the sea urchin

TABLE 7
Feeding Habits and Food of the Predominant Intertidal Xanthid Crabs of Enewetak*

Feeding habits

Food; specific foods (see

key below) in approximate

order of importance

Species

Wiien

Where

How

What

Low Intertidal Species

Liocarpllodes biunguis

Night

In open

Scraf)es thin (1 mm) algae
from rock; pulls bunches
of algal filaments away
from rock

Herbivorous

Plants (1, 3, 4, 2, 5)

Eriphia scabricuh

Day +
night

In open

Pulls thick algal turf
from rock;

Omnivorous

Plants (3, 4, 1, 2, 5)
Animals (5, 6, 7, 2, 3)

Leptodius
davaoensis

Mainly
night

Tide
pools

As in Leptodius biunguis

Herbivorous

Plants (3, 1, 2)

Xantho sanguineus

Mainly
night

Tide
pools

As in Leptodius biunguis

Herbivorous

Plants (2, 3, 4, 1, 5)

Dacr^iopilumnus
rathbunae

Day +
night

In open
or feed
from holes

Scrapes thin algae from
rock around shelter
holes

High Intertidal Species

Mainly

herbivorous

Plants (1, 4, 3, 2)
Animals (8, 5)

Pseudozius cai>strus

Always under cover

No observations

Omnivorous

Plants (3, 1, 2, 5, 4)

L\;dia annuHpes
Leptodius gracilis

Mainly
night

In open

Tide
pools

Pulls hole-dwelling prey
from shelters; pries
Siphonaha from rock;
eats thick algal turf

Scraf>es thin algae from
rock

Carnivorous
Herbivorous

Animals (3, 4, 6, 2, 7, 1)
Plants (1,3)

Plants (1, 3, 4)

•Data from Havens, 1974.

Key to plant foods: 1, blue-green algae; 2, Jania; 3, Polvsiphonia; 4, Laurencia + similar forms; 5, Ceratocentrum.
Key to animal foods: 1, Foraminifera; 2, polychaetes; 3, sipunculans; 4, Siphonaria; 5, small Crustacea; 6, crabs; 7, insects;
8, mites.

INTERTIDAL ECOLOGY

151

Heterocentrotus trigonarius to be the most important her-
bivores.

On the undersides of rocks, the amphipod Melita
celericula may be primarily herbivorous. In the laboratory it
fed on the algae Valonia and Acetabulaha as well as
detritus, fecal pellets, and small conspecific individuals
(Croker, 1971).

Carnivores on Encrusting Animals

Only the abundance of polychaetes in this category has
been quantified (Bailey-Brock, White, and Ward, 1980),
and their feeding biology has not been studied. The amphi-
nomids Eur[^thoe complanata and Pseudeuri;thoe oculifera
occur in the outer or encrusting zone of the Enewetak
Island platform 40 to 70 m from shore; they are closely
related to species known to feed primarily on corals. Many
members of the numerically dominant Syllidae probably
belong to this category; some are reputed to feed on
sponges, but despite their dominance little is known of
their biology.

Chromodohs geomethca and other doridid nudibranch
gastropods probably also feed on sponges (Young, 1967).
Other nudibranchs whose food is known include Heruielh
mietta, which eats the eggs of the prosobranch Cerithium
sejunctum, and Okadaia elegans, which eats spirorbid
polychaetes after drilling a hole through the calcareous
tube with its radula (Young, 1967).

Predators

The most important predatory invertebrates at the pri-
mary carnivore level are probably gastropods of the fami-
lies Conidac, Muricidae, Mitridae, Vasidae, and Buccinidae.
The food subwebs they participate in were being studied
by the author and Paul J. Leviten at the time of the
letter's untimely death in 1980. It is hoped this study will
be concluded in the future, but preliminary results can be
mentioned here. The pattern of trophic relationships in the
food subweb in which gastropods are the primary car-
nivores (Fig. 4) suggests that species of Conus, the most
abundant genus, prey primarily on members of different
families from the other vermivorous gastropods. Within
Conus, preliminary results indicate a general pattern of
specialization on different polychaete taxa by the co-
occurring species. Most similar to Conus diets are those of
Drupa morum, which eats mainly eunicids, and D. arach-
noldes, which eats only nereids (Bernstein, 1974). As
noted above, these limpet-like muricids are better able to
exploit the more exp>osed, seaward portions of the plat-
form where nereid and eunicid polychaetes are inaccessible
to Conus. Drupa ricina eats mainly vermetid gastropods
(92% of the diet: Bernstein, 1974). Two vermivores of dif-
ferent families, Engina mendicaria (Buccinidae) and Vasum
turbinellus (Vasidae) prey on polychaetes of two families
not utilized by other predatory gastropods. Their diets are
very similar to each other, but their sizes differ strikingly;

shells of the former average 12 mm long (range 8 to 18
mm) and those of the latter, 23 mm (range 15 to 32 mm).
Finally, Mitra litterata (family Mitridae) feeds exclusively on
sipunculans.

One uncommon Conus, C. retifer, and several abun-
dant muricids prey primarily on herbivorous gastropods.
Morula granulata eats mainly cerithiids (author's unpub-
lished observations), and the diet of Thais arnnigera con-
sists almost entirely of Siphonaria normalis (Menge, 1973).
In addition, some of the species listed in Table 7
occasionally prey on molluscs.

Several xanthid crabs arc also important primary car-
nivores on windward, seaward platforms, beachrock
outcrops, and lagoon-side rocky shores (Havens, 1974).
The upf>er intertidal Lydia annulipes is primarily a preda-
tor: 84% of stomachs examined contained animal remains
and 28% contained plant food. Sipunculans and Siphonaria
normalis are the main prey organisms (Table 7), either or
both occurring in two thirds of the individuals examined. In
contrast, these organisms were not commonly eaten by the
omnivorous xanthids Pseudozius cai/strus (4%) and Eriphia
scabricula (18%) or the partly carnivorous Dacri/opilumnus
rathbunae (1%). The diets of xanthid species that co-occur
in the same microhabitats differ strikingly. For example,
where L. annulipes and P. caysfrus use the same crevices
for shelter, the former feeds mainly on sipunculans and the
latter on large crustaceans and algae (Havens, 1974).

A recent study of the macruran Panulirus penicillatus
(McCollum, 1981) has provided considerable information
on its prey. The several types of microhabitats frequented
by P. penicillatus during foraging have been noted.
McCollum's results suggest that P. penicillatus is a nearly
omnivorous predator (Table 8); its diet is very diverse and
did not differ significantly among locations studied at
Enewetak. Panulirus penicillatus crushes its prey and swal-
lows all the parts, so prey organisms with shells or other
hard structures can be enumerated from stomach contents.
Numerically, molluscs, crustaceans, and miscellaneous
items ranked about equally; polychaetes and echinoids
predominated in the last category. Although biomass esti-
mates were not possible, a measure of imp>ort^ce based
on number, volume, and occurrence indicated molluscs to
be about twice as important as crustaceans and about
eight times as important as miscellaneous items (McCol-
lum, 1981). The stomach of one female P. penicillatus con-
tained remains of 56 individual ccrithiid gastropods, 52
individuals of Strombus sp., three trochids, and one
Fragum fragum (Bivalvia); the gastropods were enumerated
by counts of opercula. Another female had eaten many
Fragum fragum, one Pinctada sp., at least two mytilids, 57
Strombus sp., 21 muricids, probably mainly Morula spp.,
nine cerithiids, including Cerithium aloeolus, one each of
Mitra cucumerina, Pusia cancellarioides, Conus sp., and
Natica sp., and unidentified gastropods. On reef areas with
more polychaetes and hermit crabs, higher proportions of
these were consumed. In general, individual stomachs con-
tained a high diversity of prey items (McCoIIum, 1981).

152

KOHN

INTERTIDAL ECOLOGY

153

TABLE 8

Major Prey Items of Panulirus

penicillatus on Three Reef

Platforms at Enewetak

Number" of prey items

Mollusca

Gastropoda

Bivalvia

Polyplacophora

Crustacea

Brachyura

Anomura

Hoplocarida

Other

Miscellaneous

Polychaeta

Echinoidea

Madreporaria

Ophiuroidea

Pisces

Algae

'Numbers in body of table are numbers of
prey taxa at left recovered from examination
of 78 P penicillatus stomachs (McCollum,
1981).

UTILIZATION OF INTERTIDAL
HABITATS BY FISHES

Windward platform surfaces dominated by a film or
thin turf of the blue-green alga Calothrix Crustacea are
grazed intensively by fishes. Miller (1983) observed about
530 fishes per hour at high tide swimming through a 3 X
5 m quadrant on the northern part of the Enewetak Island
platform. About 85% of these were herbivores, mainly par-
rot fishes and surgeonfishes. The predominant species are
Scarus frontalis. Acanthurus triostegus, and A. guttatus
(Hiatt and Strasburg, 1960; Bakus, 1967; Webb and
Wiebe, 1975; Miller, 1983). "Their teeth marks in the reef
rock provide evidence of the thoroughness with which they
crop this alga" (Wiebe, Johannes, and Webb, 1975). "The
most striking phenomenon about the reef flat is the innu-
merable toothmarks that range from the uppermost
reaches of the dead coral substratum to the outer edge of
the algal ridge, and beyond" (Bakus, 1967).

As the incoming tide covers the platform, large schools
of Acanthurus triostegus "gradually browse their way to
the uppermost reaches of the reef flat" (Bakus, 1967).
Behind them arc large schools (about 300 to 400 fishes) of
the larger A. guttatus, then numerous schools of Scarus
spp., which remain in slightly deeper water, venturing

close to shore only between mid-tide and high-tide level
(Bakus, 1967). Analyses of Scarus frontalis and S. gibbus
indicated that both species graze only on dead coral and
filamentous algae on reef rock. They consume considerable
CaC03, which is acidified in the gut and reduced in
particle size. These fishes commence feeding at first light
(about 0730 at Enewetak). Less than 4 hours later, all indi-
viduals (mean standard length 31 cm) had full large intes-
tines. Feeding continues until dusk (about 1900), at which
time most individuals contained food in the anterior diges-
tive tracts. Six hours later, all portions of all digestive
tracts examined were empty (Smith and Paulson, 1974).
These authors thus calculated transit time of food through
the alimentary tract of 6 hours.

Juvenile Acanthurus (riostegus also browse only on
algae; stomachs of adults contain mostly algae but with a
few small coral fragments. Acanthurus guttatus graze "sig-
nificant quantities of coral fragments along with benthic
algae" (Bakus, 1967). Analysis of tooth scars by Bakus
(1967) showed that small tooth marks of A. guttatus and
juvenile scarids predominate in the inner 18 m; scars of
acanthurids and scarids intermingle over most of the plat-
form; and the algal ridge, surge channels, and pools of the
outer edge have mainly scarid scars.

From his estimate of net primary productivity of blue-
green algae noted above and by estimating the biomass of
herbivorous fishes utilizing the platform, Bakus (1967) con-
cluded the Calothix Crustacea and Schizothrix cakicola syn-
thesize organic matter at a rate adequate to support the
feeding activities of the fishes. These are, however, time-
limited by periods of high tide and probably cannot meet
their entire energy requirements by feeding only on the
windward platform.

Other herbivorous fishes on seaward platforms at high
tide are the browsing rabbitfish Siganus argentatus and,
near the surf-swept outer edge, the surgeonfishes
Acanthurus achilles and Zebrasoma veliferum (Hiatt and
Strasburg, 1960).

That the limitation of algal cover on the more barren
regions of the platforms is due to grazing has been demon-
strated by Miller (1983) who reported 100% coverage of
the platform surface by macroscopic algae after 3 months
under 5-mm mesh cages that excluded grazing fishes and
crabs but not smaller herbivorous invertebrates. Inver-
tebrate abundance also increased in the exclosures.

Algal cover grades strikingly from the thin film of
Calothrix and Schizothrix at the north end of the Enewetak
Island seaward platform to a turf of erect, macroscopic
Jania. Sphacelaria, and other forms about 300 m south.
There fish grazing intensity is only about 30% of that
observed farther north. The southern portion is probably
less accessible to herbivores because of its greater distance
from suitably sheltered subtidal sites required by the fishes
at low tide, such as the quarry on the north part of the
platform and the lagoon (Bakus, 1967; Kohn and Leviten,
1976; Miller, 1983).

Omnivorous fishes on the seaward platforms include
the common blenny Istiblennius coronatus, which eats sur-

154

KOHN

face sediment, filamentous algae and the foraminiferan,
Calcarina: the triggerfish Rhinecanthus aculeatus (main
foods: algae, gastropods, isopods, crabs, shrimp,
polychaetes, fishes); and the damselfish Abudefduf sor-
didus (main foods: algae, crabs, fishes, polychaetes,
Calcarina) (Hiatt and Strasburg, 1960).

Use of intertidal windward platforms by carnivorous
fishes at high tide is poorly documented. The black-tip reef
shark Carcharhinus melanopterus is frequently seen there
but constituted less than 0.5% of all fishes observed by
Miller (1983). This species is piscivorous (Hiatt and Stras-
burg, 1960).

UTILIZATION OF INTERTIDAL
HABITATS BY BIRDS

Shorebirds of several species fly to the windward inter-
tidal platform to feed at low tide. Johnson (1979) reported
that whimbrels fNumenius phaeopus), bristle-thighed cur-
lews (N. tahitiensis) and wandering tattlers (Heteroscelus
incanus) use intertidal habitats more intensively than the
other common Enewetak shorebirds, golden plovers (Pluui-
alis dominica fulva) and ruddy turnstones (Arenaha
interpresj. However, 1 have observed several occasions
when golden plovers were the only common birds on the
windward platform at Enewetak Island.

Bristle-thighed curlews are known to eat the proso-
branch gastropod Nerita sp. by picking up the snail in the
tip of the beak, raising the head, swinging the bill laterally
and then across the back, and finally hurling the snail
downward against the rocks. This procedure may be
repeated several times until the shell is broken; the bird
then extracts the snail's body (Carpenter, Jackson, and
Fall, 1968).

BEACH AND SUPRALITTORAL
FRINGE HABITAT

The most conspicuous invertebrates of the uppermost
intertidal and supratidal beaches are ghost crabs of the
genus Oci^pode. Two species occur, O. cordimana extend-
ing from just below high-tide line to well up in the zone of
fringing beach vegetation, and O. ceratophthalma,
ranging downward from about high-tide line. The latter
species is more common, but both tend to occur on the
same beaches. Both live in burrows during the day and are
active nocturnally. Little is known of their ecology, but at
night O. cordimana usually sits near its burrow entrance,
retreating within at the slightest disturbance. Only
O. ceratophthalma wanders over the beach at night. In the
account from which the preceding information was taken,
Horch (1975) compared acoustic and other aspects of the
behavior of these species.

In the wave-washed zone of sand beaches in Enewetak
Lagoon, the predatory anomuran crustacean Hippa pacifica
is prominent nocturnally. Mysid crustaceans, caught with
hairs on the long first pereiopods, are its main food at
Enewetak (Wenner, 1977).

Among the few meiofaunal taxa reported from
Enewetak are tardigrades of the genera Hypsibius and
Macrobiotus, found in supralittoral fringe beach sand
(Mehlen, 1972).

EFFECTS OF ENVIRONMENTAL
DISTURBANCES ON INTERTIDAL BIOTA

In recent years several cases of mass mortality of tropi-
cal intertidal organisms from severe storms or other
episodic catastrophes have been documented. These are
cited by Leviten and Kohn (1980), who described the
effects on gastropod populations of an unusually severe
rainstorm that coincided with a low tide that left the inner
40 m of the windward platform at Enewetak emcrsed for
several hours. On Sept. 3, 1972, 4.3 cm of rain fell dur-
ing a 6-hour period. A strong smell of rotting organisms
persisted for more than a week after the storm during low
tide periods, attesting to the death of many types of organ-
isms. Of 155 individuals of six species of Conus censused
in the area on Sept. 6, we found that 70% (83/119) of
C ebraeus and 92% (33/36) of the five other species
were killed. Thus the species whose distribution normally
extends farthest inshore, C ebraeus, had the highest sur-
vivorship. The nearly total mortality of all other species in
the affected area suggests that unpredictable catastrophes
such as rainstorms may prevent them from occupying
inshore areas of bench. Mortality of C ebraeus was also
size-selective: all individuals <15 mm long were killed,
while 38% of those >15 mm survived (not 15%, as
erroneously reported in Leviten and Kohn, 1980). Thus
the observed size-frequency distributions of Conus sp>ecies
noted above may also be determined by variations in
physical stress across the platform (Leviten and Kohn,
1980).

It was also possible to assess the effects on inter-
tidal gastropods of another severe environmental distur-
bance, Typhoon Alice, which struck Enewetak Jan. 5,
1979. On the central portion of the Enewetak Island plat-
form, where thick algal turf had provided protected sites
for gastropods, the turf was much thinner and population
density of Conus species was much lower after the
typhoon. However, on the portion of the platform where
cracks, crevices, and rubble-filled depressions on otherwise
smooth, bare bench were the main refuges, there was no
significant reduction of Conus abundance or species rich-
ness. Predatory gastropods other than Conus species,
predominantly (94%) Muricidae, were not significantly
reduced in number of individuals or species in the latter
area and were reduced less than Conus species in areas
with algal turf, probably due to the greater tenacity of
muricids as described above (Kohn, 1980). Predatory gas-
tropods on the windward reef platform are thus
behaviorally adapted to use refuges that shelter them ade-
quately from the most severe storm conditions likely to be
encountered there. In six of 10 comparisons involving
C. ebraeus. C. chaldaeus, and C. sponsalis at several sites,

INTERTIDAL ECOLOGY

155

size-frequency distributions were shifted significantly
toward larger shell lengths after the typhoon, suggesting
size-selective mortality with smaller individuals more likely
to die, as in the case of rainstorm-induced mortality.

CONCLUSION

The intertidal zone, populated almost exclusively by
plants and animals of marine origin, exposes these organ-
isms to the harshest physical conditions and widest ampli-
tudes of fluctuating physical variables of any oceanic
environment. On an atoll, heating and desiccation from
tropical insolation, inundation by heavy rain, and storm
waves exacerbate even these stringent conditions.
Nevertheless, complex and highly productive biotic com-
munities characterize the intertidal comfx>nent of atoll
ecosystems. At Enewetak, studies over the past 25 years
have documented the major outlines of community organi-
zation and have revealed some important, unexpected
characteristics. This chapter has summarized the
knowledge they have provided of the identity of the major
organisms present, their population densities, distribution
patterns, temporal variations, habitat requirements, and
trophic roles and interactions. However, a satisfactory syn-
thetic model of intertidal community structure and
processes will require more intensive future studies of
trophic dynamics and of both biological and physical fac-
tors affecting the composition and relationships of the
biota.

Note Added in Proof

Recent analysis of the microhabitats and diets of a
large sample of three species of Drupa collected by Paul
Leviten on the seaward, windward platform in 1972-74
amplifies the study of Bernstein (1974) reported in the text
(Thomas and Kohn, 1985).

Drupa morum. the largest species (mean shell length
25 mm) and the one occupying the most exposed, sea-
ward microhabitats, preys primarily (65% of diet) on nereid
polychaetes (Cerafonereis mirabilis and Pehneresis singa-
poriensis) and secondarily (23%) on eunicid polychaetes
{Eunice afra and Li/sidice collaris). Drupa hcinus (mean shell
length 22 mm) is more widely distributed across the plat-
form. In its more exposed microhabitats it preys primarily
on vermetid gastropods (Dendropoma gregaha) and per-
haps other species of the genus. In inner, more protected
areas it preys more frequently on nereids, mainly C mira-
bilis. Overall its diet comprised 44% vermetids, 42%
nereids, 5% other polychaetes, and 9% crustaceans. Drupa
arachnoides (mean shell length 20 mm), the most inshore
species, preys almost exclusively (92%) on C. mirabilis.
Overall, predator size and prey size were positively
correlated.

untimely death from cancer in 1980 at age 36 deprived
tropical marine ecology of a gifted and productive scientist.
Leviten was educated at the Universities of Miami (B.S
and M.S.) and Washington (Ph.D.). He was a Queen's Fel-
low for 2 years in Australia, and he served on the faculties
of the University of California at Irvine and Santa Barbara.

Fig. 5 The late Dr. Paul J. Leviten at work in the Mid-
Pacific Research Laboratory, Enewetak.

Leviten was the sole author or co-author of four major
papers, all of which repKDrted research performed at
Enewetak. His research blended mastery of theory, quanti-
tative field ecology, and devotion to knowledge of the
Mollusca in harmonious proportions, and his accomplish-
ments contributed significantly to all of these. This chapter
is therefore dedicated to his memory.

ACKNOWLEDGMENTS

Support by NSF Grants DEB 77-24430 and 81-17945,
and logistic support from ERDA Contracts AT(29-2)-26
and AT-(26-l)-628, arc gratefully acknowledged. I thank
the following directors for providing research facilities and
encouraging my research at Enewetak: R. W. Hiatt,
S. V. Smith, E. S. Reese, P. Helfrich. I thank J. T.
Harrison for discussion and criticism.

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DEDICATION

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156

KOHN

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Chapter 9

Reef Processes: Energi; and Materials Flux

JAMES A. MARSH. JR.

Marine Laboratory/, Uniuers/ty of Guam
Mangilao, Guam 96923

INTRODUCTION

A number of significant studies of reef community
processes have been conducted at Enewetak. These stud-
ies have made major contributions to an understanding of
that particular system. Their significance also lies in the
development of methodology and of a general approach to
understanding whole ecosystems. Such studies have
strongly influenced the context in which many ecologists
think about reef systems and have probably had a broader
influence on ecology generally.

One of the earliest and most important studies, con-
ducted during the first year of Enewetak Marine Biology
Laboratory (EMBL) operations, was that of Odum and
Odum (1955). This was a remarkable attempt to look at
the reef ecosystem as a whole and to relate structure to
function. It has been widely cited not only by reef
researchers but also by other ecologists and has had an
impact on ecology generally. It generated great interest
both for its approach and for its specific findings and con-
clusions. The Odums did an impressive amount of work
during their 6 weeks at the field site and then used this as
the basis for far-reaching extrapolations. Additionally, they
stimulated a great deal of interest in reefs as ecosystems
and prompted many other researchers to undertake further
studies, apparently if only to prove the Odums wrong in
some cases. Their seminal study thus occupies a central
position in a chapter on ecosystem processes of Enewetak.

A more recent study utilizing the same approach was
conducted by the SYMBIOS team in 1971 (Johannes et
al., 1972). The team, consisting of some 25 scientists with
a variety of diverse but related interests, spent 2 months
studying transects near the earlier Odum transect. This
much larger scale effort confirmed and extended many of
the original findings of the Odums in a repeat demonstra-
tion of the utility of the upstream-downstream sampling
methodology and particular processes and phenomena.

In addition to community metabolism, other important
studies to be discussed in this chapter have focused on cal-
cification processes at the ecosystem and organismal level,
on nitrogen and phosphorus cycling, on the role of detritus
(coral mucus and algal fragments), on coral nutrition, and
on ecological relationships of reef fishes. Noteworthy
research gaps include the lack of attention directed to eco-
logical relationships involving humans and the apparent
dearth of information on the impact of atomic testing, even
though EMBL and the operations it evolved into have been
supported by the Atomic Energy Commission and its suc-
cessor organizations.

Several other papers resulting from work at Enewetak
are often cited and have had a major influence on the
thinking of reef scientists. These include papers by Goreau
(1959), Hiatt and Strasburg (1960), and Muscatine (1967).
Other papers of general interest are those by Knutson et
al. (1972), Smith (1973), and Hobson and Chess (1978).

It is noteworthy that the work supported at EMBL and
its successors includes few general descriptive studies.
There are perhaps two reasons for this. First, much of the
descriptive information that is available was accumulated
incidentally during the course of other types of studies.
Second, and perhaps more important, much information
was previously available as a result of extensive surveys
(some of them quantitative) during Operation Crossroads.
Hence, much of the descriptive background for more func-
tional studies was already available when EMBL began
operations. Nevertheless, additional descriptive informa-
tion, with an emphasis on quantitative observations, would
probably be helpful.

The large number of geological, physical, and geochem-
ical studies carried out at Enewetak makes this one of the
most thoroughly studied reef systems in the world. It is
ironic that much of this information is still apparently scat-
tered in various sources, especially unpublished ones, and
perhaps available only in the files of different funding agen-
cies. A careful synthesis of such potentially available infor-
mation might lead to a more comprehensive overview of
the Enewetak ecosystem than is otherwise possible — an
overview based on more than an "expedition" mentality. It
is thus unfortunate that the ecosystem with the greatest

159

160

MARSH

potential for an integrative understanding of reef
processes, and the greatest realized development of such
an understanding to date, has not seen this potential fully
developed.

Most of the studies of Enewetak processes have
focused on reef flats rather than deep faces or lagoons.
Indeed, "reef" is synonymous with "reef flat" for many
researchers. There is some justification for this position
since this is the portion of the reef system that receives
the greatest inputs of solar energy required for primary
productivity and has been shown to be the most actively
calcifying portion of the system (Smith and Harrison,
1977). It also appears to be the major site of nitrogen fixa-
tion (Wiebe, 1976). Finally, at least as a first approxima-
tion, the reef flat appears to be the source of the major
nutrient and energy inputs to the lagoon, the most exten-
sive subsystem of the atoll. Indeed, the impression shared
by many reef ecologists is that the comparatively small,
intensely productive reef flats provide the major inputs
driving the whole system.

DESCRIPTIVE STUDIES OF
REEF STRUCTURE

The general picture that many f)eople have of biologi-
cal zonation on reefs, especially Pacific atoll reef flats, is
strongly influenced by the description provided by Odum
and Odum (1955). They described a series of zones
extending roughly parallel to the breaking surf on the
windward reef margin and perpendicular to the direction of
water flow across the reef flat. They also provided esti-
mates of the standing crops of dominant reef organisms.
This built upon extensive earlier descriptions by Tracey et
al. (1948) and Emery et al. (1954). The former paper pro-
vided a general system of classification for various reef
types and made the distinction between island reefs and
intcrisland reefs on atolls. The Odums' description applies
generally to at least some fringing reef flats (e.g.. Marsh,
1974) as well as to atolls. Their work still stands as a use-
ful general description of reef flats.

The Odums distinguished six zones on their reef,
proceeding from seaward to lagoon. The windward buttress
zone constitutes the seaward face of the reef outside
breaking surf; it was inaccessible to the Odums but they
estimated that there is about 50% coral coverage on the
submarine buttresses in this zone. The coral-algal ridge
zone is dominated by calcareous red algae and fleshy algal
mats, with scattered encrusting forms of Acropora. Pocillo-
pora. and Millepora corals. Behind this the encrusting zone
likewise has sheets of yellow encrusting Acropora and
Millepora and low, rounded heads of Porites and several
species of favid corals. Filamentous algae of all four major
benthic algal divisions form heavy encrusting mats here.
Coral cover is far less than 50%. Next is the zone of
smaller heads with massive heads of Porites lobata and
favid corals; encrusting Acropora is not present but scat-
tered colonies of branching corzils of the genus can be

found. The zone of larger heads is slightly deeper and has
massive heads up to a meter high and 2 m across, with
sand channels between the heads. The blue octocoral
Heliopora is common here, with a distinct narrow zone of
the stinging coral Millepora at the back edge of the zone of
larger heads. Parrotfishes and surgeonfishes commonly
browse and school here. Algal cover is much lower in the
zones of smaller and larger heads than in the two zones
immediately upstream. The zone of sand and shingle has
very low occurrences of either algae or corals and few of
the fishes found in the upstream zones; however, there arc
schools of sardine-like fishes that feed on "pseudoplank-
ton" (algal fragments) drifting downstream in the current.
To the Odums, the reef structure suggested a transition
from water-filtering as a source of nutrients in upstream
zones to subsurface decomposition as a source of plant
requirements in back reef zones.

Odum and Odum (1955) attempted to get biomass
estimates of the different trophic levels for different reef
zones. This was an ambitious undertaking that has not
been repeated, presumably because of the large amount of
work involved and the uncertainties of assigning particular
organisms to specific trophic levels. An attempt to repeat
their estimates in the light of more recent information on
the basic biology of the organisms involved is obviously
called for in a variety of reef ecosystems.

The Odums estimated the dry weights of primary pro-
ducers by chlorophyll extractions (based on Harvey pig-
ment units) calibrated by establishing a ratio of chlorophyll
to dry weight for the free-living macroalga Codium. This
was done for free-living algae of various growth forms, for
zooxanthellae contained in the living tissue of coral polyps,
and for filamentous green algae ("boring" algae) within the
skeletons of corals and other calcareous material. The
mean estimate for producers averaged over all reef zones
was 703 g dry biomass m~ , with little evident difference
in the producer component of live corals from different
reef zones. The white sand area of the back-reef zone was
found to be the only major reef zone with a definitely
lower biomass of producer organisms. From data on colo-
nization of glass sides left on the reef, the Odums calcu-
lated the growth of encrusting algae on the front reef to be
twice that on the back reef, consistent with the observed
predominance of encrusting forms in the former zone and
boring forms in the latter zone.

Animal biomass at the primary consumer level (second
trophic level) was reported by the Odums to consist pri-
marily of fish, coral polyps, molluscs, echinoderms,
annelids, and crustaceans, depjending upon the particular
reef zone. The measured biomass averaged 132 g dry
weight m~^ for all reef zones combined. The third trophic
level was found to consist primarily of predatory fish, mol-
luscs, crabs, annelids, and starfish, averaging 11 g dry wt
m~^. The ratio of herbivores to producers was calculated
to be 18.9% and of carnivores to herbivores, 8.3%. This
resulted in a pyramid of biomass with a broad base and a
small peak, a result stressed by Odum and Odum as being
consistent with ecological theory.

REEF PROCESSES

161

COMMUNITY STUDIES

Oxygen Metabolism and
Primary Productivity

The seminal nature of the Odum and Odum (1955)
study has been noted. Theirs was not the first upstream-
downstream study of reef metabolism, being preceded by
that of Sargent and Austin (1949, 1954). It succeeded,
however, in generating widespread interest and is probably
responsible for the frequently made statements that reef
systems are oases of high productivity surrounded by
nutrient-poor deserts, that they are among the most highly
productive systems on earth, and that they are highly effi-
cient transformers of solar energy into biological energy.
The Odums' work was also responsible for promoting the
knowledge that it is the benthic community rather than the
phytoplankton in the overlying water which is responsible
for this high productivity. These are all key elements in
our present understanding of reef ecosystems.

More precise and thorough measurements of commu-
nity metabolism were carried out by the Project SYMBIOS
team (Johannes et al., 1972; Smith, 1973; Smith and
Marsh, 1973), but the basic conclusions reinforced those of
the Odums regarding the high productivity of the wind-
ward reef flat. The team further provided at least a partial
answer to the commonly asked question of how reefs
could be areas of such high productivity surrounded by
nutrient-poor and unproductive oceanic waters. It con-
cluded that high rates of nitrogen fixation and extremely
efficient internal recycling of phosphorus were major fac-
tors. This conclusion was later disputed by Atkinson
(1981), whose work is discussed in the section on com-
munity phosphorus cycling.

Smith and Marsh (1973) made simultaneous measure-
ments of oxygen and carbon dioxide changes in water as it
flowed across the windward reef flat. The two independent
estimates gave strongly correlated results for daytime pro-
ductivity values but a weaker correlation for nighttime
respiration values. These results allowed them to make the
first estimate of the metabolic ratio (i.e., the molar change
in CO2 per molar change in oxygen) for a reef community.
They found this ratio to be —1, uncorrected for diffusion,
and suggested that corrections for diffusion without some
indication of the metabolic ratio did not increase precision
of productivity measurements.

Smith and Marsh comjjared two reef-flat transects, one
similar to that described by Odum and Odum and contain-
ing coral zones as well as algal zones, and one transect of
similar length but crossing no coral zones. The algal tran-
sect had a much higher gross P, net P, and gross P:R ratio
(based on a 24-h period) but had similar rates of respira-
tion, perhaps reaching an upper limit imposed by diffusion
rates of oxygen to the large benthic infaunal community.
Their coralgal transect had an overall gross P;R ratio near
1 and was apparently just self-sufficient with resF>ect to
energy demands. Assuming, however, that the algal por-
tion of the coralgal transect was metabolically similar to

the algal transect. Smith and Marsh calculated that the
coral portion of the former transect was heterotrophic.
Thus, there appeared to be an upstream autotrophic por-
tion and a downstream heterotrophic portion for that tran-
sect. They postulated that the large schools of herbivorous
fishes migrating between reef zones could be significant in
transferring energy and materials downstream. Perhaps
more attention should be directed to the question of
whether this distinction of an upstream autotrophic and a
downstream heterotrophic zone is a general feature of
reef-flat ecosystems, as originally suggested by Odum and
Odum. Much more comprehensive studies of community
metabolism, especially of reef flats, have been made at
other study sites by Kinsey (1977, 1979).

Wells (1974) and Wells et al. (1973) described a
method for making in-situ measurements of benthic
metabolism in reef communities and presented some pre-
liminary results. Their basic technique was to place a trans-
parent plastic dome over a suitable portion of the substra-
tum and to monitor oxygen changes in the enclosed water
mass in the light and in the dark. Preliminary measure-
ments were made at Enewetak on the SYMBIOS transect
and at sites in the Virgin Islands. At Enewetak, enclosed
water masses were reported to show a rather constant
oxygen concentration of 125% saturation while gas bub-
bles were being produced in the light; the oxygen content
of the evolved gas was 28 to 32%. The oxygen evolution
of algae-covered pavement reached a maximum of 5.5 X
10~ ml cm~^ h"~^ with a maximum P:R ratio (24-h basis)
of 1.6. Coral rubble was observed to produce at about half
the rates of the algae-covered pavement. These prelim-
inary attempts to measure metabolic activity of the algal
pavement thus focused on a neglected, but probably
major, component of the reef-flat ecosystem. Marsh like-
wise has made a few unpublished measurements of small
sections of such pavement removed from the reef and
placed in respirometers. His preliminary values for gross P
ranged up to 0.087 mg O2 cm~^ h"^ and suggested that
this might be one of the most metabolically active seg-
ments of the reef-flat ecosystem; this should be followed
up.

Calcium Carbonate Production and
Reef Growth

While there were several earlier attempts to estimate
the growth rates of individual reef components, especially
corals (e.g., Mayor, 1924), one of the first attempts to
assess calcium carbonate deposition directly for the reef
community as a whole was made by Smith (1973). Along
with concurrent work by Kinsey in Australia (1972), this
research pioneered the technique of utilizing changes in pH
and alkalinity as water flowed across a reef flat not only to
assess organic productivity but also to evaluate the dynam-
ics of calcium carbonate deposition and dissolution at an
ecosystem level. Smith found that both a coralgal transect
and a transect dominated by an algal turf calcified at an

162

MARSH

average rate of 4000 g CaCOa m^^ yr^'. There were no
apparent differences between day and nigfit in these
studies. Smith further calculated that, although there was
little particulate CaC03 removal from the reef flat over the
duration of his studies, there has been virtually no net
CaCOs accumulation on the windward reef flats of
Enewetak over the last several thousand years. He thought
that lagoonward accumulation is the probable sink for cal-
careous material produced on the reef flat but that sedi-
ment transport occurs almost exclusively during periods of
intense wave action.

The observation that daytime and nighttime calcifica-
tion rates for the whole community are similar ran counter
to much previous thinking, which was strongly influenced
by the measurements of individual organisms enclosed in
small containers. For instance, Goreau (1961) reported
that calcification rates in individual corals, as measured by
uptake of ''^Ca, were strongly light-dep>endent. However,
Smith pointed out, as had others before him (e.g., Chave
et al., 1972), that there are large uncertainties inherent in
using the standing crop and turnover of individual organ-
isms to estimate CaCOs production of the community as a
whole. Furthermore, it is likely that corals, which are most
often used in the individual-organism approach, account for
a minor component of total calcium carbonate production
on a reef. Smith also pointed out that the technique of
measuring alkalinity depletion as a way to estimate CaCOa
deposition could be applied in incubation chambers with
individual organisms. Smith (1974) stated in a later paper
that community precipitation of CaCOs, ranging from
— 0.02 to 0.2 moles CaC03 m~^ d"\ is an order of
magitude lower than the calculated CO2 flux resulting from
organic carbon metabolism (±0.2 to 6 moles d^'),
although the former process is not reversible on a
day-night cycle and the latter process is. Flux due to diffu-
sion across the air-sea interface or to mixing of water
masses is likewise an order of magnitude lower than that
resulting from organic carbon metabolism.

Smith and Kinsey (1976) combined the data and ideas
generated by the Enewetak research with those from reef
research elsewhere to make some generalizations about
calcium carbonate production and sea level change. They
suggested that shallow, seaward portions of most modern
reefs produce approximately 4 kg CaCOs m~^ yr~\ and
that "protected" areas produce about 0.8 kg. They argued
that the difference in these rates is probably due largely to
differences in water motion. The more rapid rate is
equivalent to a maximum vertical accretion of 3 to 5 mm
per year and places an upper limit on the potential of
modern coral reef communities to create a significant verti-
cal structure during a rising sea level. They suggested that
the major taxa accounting for most CaC03 deposition
rates are corals, coralline red algae, and calcareous green
algae; the potential reef accretion, however, does not
appear to be affected by coral versus algal dominance.
They found little evidence for latitudinal gradients.

Smith and Harrison (1977), using dome enclosures
placed over the benthic community and following pH and

alkalinity changes in the enclosed water mass, assessed cal-
cium carbonate production of the mare incognitum on the
upper seaward reef slope, a habitat barely considered in
any previous study of any type. They made measurements
on "vasiform" Acropora heads and on algal pavement but
not on sand and rubble substrata. Smith and Harrison
reported that calcium carbonate production by the corals is
substantially lower on the seaward slop>c than at the con-
trol site (a subtidal quarry on the reef flat of Enewetak
Island) and that it may decrease with depth. Production by
algal pavement was also reported to be lower on the slope
than at the control site but showed no apparent reduction
with depth. However, calcium carbonate production by
algal pavement in the quarry was dramatically slower than
that by algal pavement on the reef crest. The coral calcifi-
cation rate (on a square-meter basis) was always greater
than that for algal pavement by a factor of 3 to 9. As in
other reef habitats, it was recognized that topographic in-
equalities of the mare incognitum increase the effective sur-
face area by up to 50%. Smith and Harrison concluded
that the most actively calcifying portion of an atoll is near
sea level, even though standing crops of calcifying organ-
isms on the reef flat may be lower than on the reef slope.
This is consistent with assumptions inherent in earlier
studies that the major metabolic activity is on the reef flat
rather than in other subsystems of the atoll. Their
presumed optimum environment for reef development is a
broad shoal area only a few meters deep with exposure to
oceanic swells.

Nitrogen Flux

It was Odum and Odum (1955) again who first con-
sidered nitrogen flux on reefs and made a few measure-
ments of changes in nitrate and ammonium as waters
flowed across the windward reef flat. Gilmartin (1960) like-
wise made a few measurements in lagoon waters by "stan-
dard oceanographic techniques," but the first extensive
measurements of any nitrogen compounds were made dur-
ing Project SYMBIOS (Johannes et al., 1972; Webb et al.,
1975). The researchers observed changes in various nitro-
gen species as water flowed across the usual coralgal or
exclusively algal transect and found that both transects
showed a significant net export of combined nitrogen,
implying a large input of nitrogen into the system from a
source other than combined nitrogen in incoming waters.
Following up on this observation, they found that there
were high rates of gaseous nitrogen fixation in the reef
ecosystem, the first time that this process had been
reported for any such system. The transect dominated by
algae showed a net uptake of nitrate-nitrogen, and there
was a net export of that species from the coralgal transect.
Other nitrogen species (ammonium, dissolved organic nitro-
gen [DON], and particulate organic nitrogen [PON]) like-
wise increased significantly in waters flowing across this
transect. The DON concentrations (2300 to 3000 nmol)
were about an order of magnitude higher than the PON
values. On this transect, there was a net removal of N03~

REEF PROCESSES

163

on the alqal portion and a net release on the coral portion.
There were no significant day-night differences, but there
was greater export of NH4^, DON, and total N during
noon-to-midnight periods than during midnight-to-noon
periods. The C:N ratio decreased progressively from
offshore (15; 1) to the lagoon (6.6:1). There was a major
input of organic nitrogen to downstream portions of the
coralgal transect, mainly in the form of algal fragments bro-
ken off from the surf zone. The blue-green alga Calothrix
Crustacea seemed to be a major nitrogen fixer; hetero-
trophic bacteria were apparently not important in this pro-
cess since fixation was strongly light-dependent. The
increase in POC as water crossed the reef flat was propor-
tionately less than that of total nitrogen. Webb et al. calcu-
lated that 1000 kg N ha"^ yr~' was exported from the
reef flat, a high value that falls at the upper end of the
range of nitrogen fixation values for managed agricultural
plots. As we have already seen, this process of nitrogen
fixation was invoked by Johannes et al. as an important
part of the explanation of how reef ecosystems have high
productivities in the midst of nutrient-poor oceanic waters.
Nitrogen metabolism, like other metabolic processes, was
clearly found not to be dominated by corals.

Webb and Wiebe (1975) made additional observations
on the nitrification processes on a reef; they reported on
in-situ and in-vivo incubations with and without an
ammonium oxidase inhibitor and concluded that an auto-
trophic pathway involving two separate organisms was
operating in the oxidation of ammonium to nitrate. The
bacterium Nitrobacter agilis was found to be at least one
organism responsible for the terminal oxidation of NO2 to
NO3-.

A later paper, by Wiebe et al. (1975), also considered
aspects of nitrogen fixation in a coral reef community. It
suggested that, since algal flats fix nitrogen at rates com-
parable to those in managed agriculture, and this fixation
contributes to high productivity of adjacent reefs and
lagoons, algal flats should receive increased conservation
priority. They further observed that Calothrix Crustacea,
the dominant nitrogen fixer, grows in two forms. One of
these forms is a thin, yellow-brown, often almost
unispecific film covering large portions of the intertidal reef
flat. Most of this algal film remains moist at low tide; at
high tide herbivorous fish (especially acanthurids and
scarids) graze it extensively. Another growth form of
C. Crustacea occurs along the upper intertidal bench zone
as a black, felt-like mat up to 5 mm thick; this mostly dries
out at low tide and is not heavily grazed by herbivorous
fish at high tide because the water is too shallow. The
nitrogen fixation rates of moist samples of the upper inter-
tidal form averaged only 60% of those of the reef flat (34
versus 55 X 10~^ moles h~^ cm~^). However, per unit
of horizontal map area, the actual surface area of coral
and reef rubble is much greater than that on the algal flat
and may, therefore, lead to comparable rates of nitrogen
fixation for the two habitats, normalized to square meters
of map area. Wiebe et al. further stated that the nitrogen
fixed by Calothrix may enter the reef trophic web directly

by grazing, through broken-off fragments, or through
release into solution.

Wiebe (1976) summarized the above studies and
further pointed out that salinities ranging from 2 to 45 ppt
had no detectable effect on the rate of nitrogen fixation.
Furthermore, the rate was temperature-dependent and
approximately doubled between 27°C and 36°C, was 0 at
24°C, and increased for 2 hours then ceased at 39°C.
The greatest upstream-downstream increase in concen-
tration of nitrogen species was for DON, followed by NH3
and NOs^ in about equal concentrations. Not detected in
the flow studies was NO2 ■

Phosphorus Cycling

There are fewer studies of phosphorus cycling in the
reef ecosystem as a whole. Odum and Odum (1955) made
a few measurements of reactive phosphorus in waters flow-
ing over the intcrisland reef, in waters entering the wide
passage not far from their reef transect (presumed to be
representative of oceanic waters outside the atoll) and in
the lagoon. They reported levels of 0.26 to 0.64 ^g atoms
1~' and concluded that there was a tight cycling of this ele-
ment internally in the reef-flat community. Gilmartin (1960)
likewise made a few measurements in lagoon waters and
found generally low levels of the same order of magnitude
as those reported by the Odums. Measurements by
Pomeroy and Kuenzler (1967) were made incidentally to
work with individual populations; their work is discussed
later.

The first extensive measurements of changes in waters
flowing across the reef flats were made by Pilson and
Betzer (Johannes et al., 1972; Pilson and Betzer, 1973).
They reported that the concentrations of reactive and
organic phosphorus did not show detectable change in
waters flowing across a coralgal transect but that there was
a slight decrease in reactive P and a slight increase in
organic P across a strictly algal transect. In particular, they
found that concentrations did not vary in proportion to
photosynthesis and respiration rates of the whole commu-
nity, despite the fact that their ability to detect changes
was at least two orders of magnitude more sensitive than
would have been required to detect such changes if the
Redfield (atomic) oxygen:phosphorus ratio of 138:1 was
applicable in this system. Pilson and Betzer also found no
diurnal variations in concentrations as waters flowed across
the repf. This remarkable constancy suggested to them
that the plants were taking up phosphorus at a nearly con-
stant rate, regardless of the magnitude of photosynthetic
activity. Their mean concentrations of P in flowing waters
were 326 nmoles total P, 172 nmol reactive P, and 154
nmol organic P.

The conclusions of Pilson and Betzer were later chal-
lenged by Atkinson (1981), who worked primarily in
Kaneohe Bay, Hawaii, but also made some observations at
Enewetak. He found that exchange rates between reef
benthos and the water column did not fit the Redfield ratio
and concluded that changes in phosphate concentration of

164

MARSH

waters flowing over the reef flat could be used as an indi-
cator of community metabolism. He further argued that
"recycling of phosphorus for a whole reef flat is not tight,
and that the system can depend primarily on exchange
with the water column for its nutrients."

Role of Regenerative Spaces

One aspect of ecosystem-level processes that has prob-
ably not received sufficient attention is the role of the
extensive internal spaces of the reef in nutrient regenera-
tion. DiSalvo (1969, 1971), in work conducted partly at
Enewetak and partly in Kaneohe Bay, Hawaii, recognized
the potential importance of these spaces and attempted to
assess some of their quantitative aspects. He obtained bac-
terial counts of 10'' to 10^ bacteria per g dry wt and found
that some isolated bacteria were capable of digesting chitin
in vitro, suggesting there might be bacterial degradation of
the organic matrix of coral skeletons (as opposed to the
CaCOa making up the bulk of the skeletal mass). DiSalvo
also obtained estimates of the oxygen demands of internal
sediments amounting to 0.06 to 0.50 mg O2 g sediment"'
h ; these rates were considerably lowered by antibiotics.
The O2 consumption of sediments in suspension was 10%
of the total consumption by two intact heads. Water
samples collected from within the regenerative spaces in
situ generally showed oxygen debts as compared with
ambient reef water. DiSalvo reported that the oxygen
debts of inshore (stressed) reefs in Kaneohe Bay were
greater than those of offshore reefs. It is curious that the
role of internal spaces, which are quite extensive in reef
systems, has not been the subject of more studies. Their
potential significance seems great, but the preliminary
work of DiSalvo has not received much follow-up.

STUDIES OF INDIVIDUAL
POPULATIONS

Studies of energy and materials flux in individual reef
populations have focused on the corals. There is a relative
paucity of information about other populations. Further-
more, there has been no attempt (other than that of Odum
and Odum) to investigate the role of individual populations
and to integrate these roles to arrive at an estimate of
whole-community function. There is thus a wide gap
between ecosystem- or community-level studies on the one
hand and individual-population studies on the other hand.
However, several studies have certainly resulted in the
implication, if not direct evidence, that the corals are a
relatively unimportant component in total energy and
materials flux; although they are visually impressive and
are generally taken as characterizing the very essence of
the reef, their role may be misinterpreted.

Algal Productivity and Growth

Marsh (1970) made one of the earliest attempts to
assess the primary productivity of an individual algal popu-
lation and evaluate its role in total reef productivity. He

worked with encrusting forms of calcareous red algae, sam-
ples of which were removed from the reef and placed in
respirometers in the laboratory. He measured an average
gross photosynthesis of 0.048 mg O2 m~^ h"' under con-
ditions of light saturation; the light-saturation intensity was
reported to be 1000 ft-candles (less than 10% of full sun-
light), with no photoinhibition at higher light intensities.
Rates of photosynthesis and respiration in flowing water
showed no correlation with different water velocities but
were greater than rates in still water. Daily patterns of
photosynthesis calculated for populations living on the sub-
marine face of atolls suggested that light is probably not a
limiting factor for that process during most daylight hours.
Marsh calculated productivity for various calcareous algal
zones and concluded that these zones do not contribute
significantly to overall community productivity. Island reefs
on atolls, where such zones account for a larger percent-
age of the total reef area, were estimated to be less pro-
ductive than interisland reefs on the same atolls. Marsh
stated that the productivity of calcareous algae is of the
same order of magnitude as values reported in the litera-
ture for other benthic producers but is near the lower end
of the range His estimates, however, were based on the
actual surface area of individual samples rather than the
flat map area (for which 1 m^ is occupied by more than 1
m of irregular photosynthesizing surface). Marsh's values
were thus underestimates as compared with those based
on flat map area.

A study by Hillis-Colinvaux (1977) focused primarily on
a field survey of natural distributions of various species of
Halimeda. However, the author combined this with infor-
mation from previous laboratory studies of photosynthesis
and respiration of Halimeda from the Caribbean (Hillis-
Colinvaux, 1974) to calculate productivity for some
Enewetak populations. In localized reef and lagoonal areas
where the coverage of these species approached 100%,
Hillis-Colinvaux calculated that their productivity could be
as high as 2.3 g C m"^ d~' and could contribute a signifi-
cant amount of energy to the total reef system. Hillis-
Colinvaux further concluded that Halimeda populations
contribute a significant proportion of the loose sediments
in the atoll system, with seven species being the main con-
tributors.

Bakus (1967) used a different approach in evaluating
the primary productivity of "dense but thin" algal mats
growing on the hard substratum of intertidal reef flats. He
scraped off all the algae in small quadrats and weighed,
combusted, and reweighed the harvested material to get
an estimate of the initial organic standing crop. After plac-
ing exclosures over the scraped quadrats to keep out graz-
ing fishes, he then repeated the scraping after 15 to 17
days to get an estimate of the productivity by the harvest
method. The dominant species of the scraped algae were
the blue-greens Calothrix Crustacea and Schizothrix cal-
cicola. Bakus' estimates of net daily production measured
by this technique ranged between 0.6 and 2.15 g C m~^.

One major conclusion of the Odums regarding the
importance of an individual population received widespread

REEF PROCESSES

165

attention but can now be rejected. They found filamentous
boring algae in almost all calcareous substrata they exam-
ined and concluded that this was a major contributor to
total primary productivity of the system as a whole. This
has been fairly conclusively disproved by the work of
Kanwisher and Wainwright (1967), Franzisket (1968), and
Halldal (1968). They presented evidence that very little
light penetrates through the outer layers of coral tissue to
the skeletal boring algae, which in turn saturate at low
light levels and have low total photosynthetic output. It is
thus unlikely that these boring algae play a significant role
in either the nutrition of individual corals or the total pro-
ductivity of the system.

All these individual population studies are preliminary,
but they are the only attempts so far to make a statement
about the contributions of particular populations to total
productivity of the Enewetak system. Thus there continues
to be a gap between whole-ecosystem studies and
individual-population studies.

Coral Nutrition, Metabolism, and Growth

Several studies conducted at Enewetak have been con-
cerned with one of the longest-running debates about any
aspect of reef biology: how corals obtain their nutrition
and the role of symbiotic zooxanthellae in meeting their
energy requirements. Studies in this area started with the
work of Yonge et al. on the Great Barrier Reef Exp>edition
of 1929, and the ensuing debate of whether corals are
"autotrophic" (i.e., obtaining all their energy requirements
from their symbiotic algae) or "heterotrophic" (i.e., depen-
dent on the capture of zooplankton for at least part of
their energy requirements) continues to this day.

A significant contribution was made by Muscatine
(1967), who demonstrated that zooxanthellae isolated from
reef corals and giant clams incorporated radioactively
labeled CO2 during photosynthesis. In the presence of
some component of host tissue, up to 40% of the labeled
algal photosynthate was liberated from the algal cells, pri-
marily as glycerol. Muscatine was unable to evaluate the
rates at which this occurred in situ but suggested that
excretion of glycerol by the algae and its control and utili-
zation by the host may represent a mechanism whereby
the zooxanthellae contribute to the productivity of reefs.
The work of Muscatine has subsequently been widely cited
and has been influential in shaping the way that research-
ers think about coral nutrition and its role in reef function,
although quantitative determinations of transfer rates are
needed.

Roffman (1968) worked with several species of intact
corals (with their enclosed zooxanthellae) removed from
the reef and placed in respirometers exposed to ambient
sunlight, to reduced light levels resulting from various
layers of screening, and to complete darkness. He calcu-
lated P:R ratios and concluded that some species have at
least the capability of obtaining all their nutritional require-
ments from their symbiotic algae. This is representative of
similar conclusions drawn from a variety of studies con-

ducted at Enewetak and elsewhere; in fact, the research
itself is representative of a commonly used approach in
dealing with the question of coral nutrition.

Wethey and Porter (1976a, b) likewise used a
respirometer approach in obtaining evidence for sun and
shade differences in corals, as reflected in variable rates of
net photosynthesis of individual colonies exposed to differ-
ences in the radiant-energy flux. Working with the folia-
ceous species Pauona praetorta, Wethey and Porter found
that colonies from a depth of 25 m had a lower 9^^ (max-
imum rate of net photosynthesis) and a lower K^, (light
level at which the intact coral-algal association photosyn-
thesized at half its maximum rate) than colonies from 10
m. In this case, net P was expressed as mg O2 mg chloro-
phyll a ' h ^ however, they did not report the amount of
chlorophyll in the colonies from the two depths. Wethey
and Porter estimated that the ratios of gross P:R for shal-
low and deep corals were 1.79 and 1.81, respectively, for
sunny days and 1.44 and 1.50, respectively, for overcast
days. A shallow-growing individual placed at 25 m was cal-
culated to have a ratio of 1.08 on overcast days. Hence,
shallow and deep colonies were considered to fare equally
well under parallel weather conditions.

Wethey and Porter also estimated the percentage of
gross photosynthesis needed to sustain the coral-algal
association for 24 hours and calculated this to be 31% and
30% for shallow and deep corals, respectively, on sunny
days and 45% and 42%, respectively, on overcast days. A
shallow colony, if placed at 25 m, was calculated to
require 68% of its gross photosynthesis on overcast days.
According to Wethey and Porter, the acclimation of deep
corals compensates completely for low available light. They
stated that the species studied is morphologically special-
ized for autotrophy and is capable of a purely autotrophic
existence down to 25 m, even under overcast conditions.
They suggested that this species has acclimated to the
worst conditions that it is frequently exposed to rather
than to the worst conditions ever encountered on an infre-
quent basis. Their work appears to have important implica-
tions for coral nutrition and is beginning to be followed up.

There has been much interest in the physiology of
skeletal formation in corals since the late fifties; important
work in this field was done at Enewetak. Goreau (1959),
in a paper which has been widely cited and which contains
a widely reproduced schematic figure of the chemical path-
ways in calcification, employed the then-new technique of
measuring Ca uptake to examine the calcification process
and factors influencing it. He reported that the rate of
uptake of radioactive calcium was significantly lowered for
corals incubated in the dark versus those incubated in the
light. Furthermore, the calcification rate of corals held in
darkness for long enough periods to cause expulsion of
their zooxanthellae was considerably reduced but was
apparently independent of light intensity. The existence of
growth gradients for different parts of coral colonies was
shown in a number of species. Calcium uptake was greatly
reduced by a specific carbon anhydrase inhibitor; but there
was still some uptake with complete inhibition, even in the

166

MARSH

dark. Goreau concluded that the effect of light on coral
growth is at least partly mediated through the zooxanthel-
lae. A few experiments were also done with the calcareous
red alga Porolithon^ Goreau's pioneering study greatly
influenced the thinking and research of later researchers.

Clausen and Roth (1975) looked at the effect of tem-
perature and temperature adaptation on calcification rates
in the coral Pocillopora damicornis^ They reported that
temperature has a marked effect on the rate of ^^Ca
uptake but that the effect varies depending upon the tem-
perature history of the coral (interpreted as meaning that
temperature adaptation occurs). The temperature optimum
shifted from 27°C to 31 °C, depending upon the tempera-
ture at which the corals had been previously held. Clausen
and Roth also noted the great variability in rates of Ca
uptake even when all experimental material and conditions
were as constant as could be achieved.

Chalker (1976) studied the mechanism of calcium
transport during skeletogenesis in the corals Acropora cer-
uicornis and A. formosa. He found that light-enhanced cal-
cification results from the active transport of calcium ions
and shows saturation enzyme kinetics. On the other hand,
dark calcification, as simulated by the addition of the pho-
tosynthetic inhibitor DCMU, results from enzyme-mediated
isotopic exchange. Strontium was found to be a competi-
tive inhibitor of both light-enhanced and dark calcification.
Chalker concluded that his data refuted the diffusional
model for calcium movement in hermatypic corals. He also
reported that light-enhanced calcification creates a signifi-
cant energy demand which may possibly be satisfied by
the oxidation of low-molecular weight compounds translo-
cated from the symbiotic algae to animal tissue. In refer-
ence to the earlier work of Muscatine and subsequent
researchers, Chalker suggested that other organic com-
pounds besides glucose, glycerol, and alanine should be
examined for such possible translocation.

Knutson et al. (1972) and Buddemeicr et al. (1974),
working at a different level of biological organization,
reported cyclic variations in the radial density of coral
skeletons, as revealed by X-radiography of thinly sliced
samples. The presence of bands of radioactivity deposited
in the coral structure by atomic testing at known dates
allowed calibration of these growth bands, which were thus
found to be annual. This "retrospective analysis" of coral
growth opened up a new area of research which was then
followed up by Buddemeier et al. The calibration pro-
cedure took advantage of the unusual situation created by
previous atomic testing at Enewetak and could not have
been accomplished at most other sites.

Knutson and Buddemeier (1973) followed up on the
initial work by examining the distributions of radionuclides
in reef corals. They reported that historic variations in the
specific activity of surface oceanic ^Sr and ^''C could be
reconstructed from band-dated colonies. Studies of the *^Sr
content of Enewetak corals suggested that the lagoon com-
munity was acting as a long-term source of that radioiso-
tope. Knutson and Buddemeier further reported that they
could detect no significant changes in coral growth rates,

patterns, or skeletal structures related to previous nuclear
weapons tests.

Highsmith (1979) studied the relationship between
coral growth rates and the environmental control of density
banding in the massive species Favia pallida, Goniastrea
retiformis, and Pontes lutea. Of these species, Goniastrea
has the densest skeleton but an intermediate growth rate;
Pontes grows more rapidly. All three species grow indeter-
minately and at a declining growrth rate with increasing
depth. Favia was found to have a linear growth rate of 5.7
mm yr^' and a mass growth rate of 0.82 g cm~^ yr ';
Goniastrea had rates of 6.8 mm yr~^ and 1.16 g cm~^
yr~', resfsectively; and Pontes, rates of 7.6 mm yr~' and
1.07 cm~ yr~\ respectively.

Highsmith found that the high-density portion of
annual-band couplets is produced during late summer and
fall when water temperature is highest and light is possibly
reduced; low-density portions of the annual couplets are
formed during seasonally lower temperatures and possibly
higher light availability In deep>er water, the high-density
portions of the skeletons account for a greater proportion
of linear and mass growth than in shallower water; the
high-density portions of the skeletons also account for a
greater proportion in those corals with slower growth
rates. This led to the prediction that linear growth will be
highest where conditions are most favorable for deposition
of low-density skeletal material. Highsmith further
proposed that matrix production in the skeleton is more
closely linked to activities of the zooxanthellae than is
extracellular calcification and that the former tends to
decline sharply at temperatures above or below the
optimum of 27°C or with decreasing light. On the other
hand, extracellular calcification is [Xjsitively correlated with
temperature, at least up to 31°C to 32°C.

Nitrogen Flux in Individual Populations

Interest in the nitrogen flux of individual populations
has been slower to develop than interest in oxygen metab-
olism and calcification and has followed the previously dis-
cussed studies of nitrogen flux in reef communities as a
whole. Aside from nitrogen-fixing algae and bacteria,
interest has focused primarily on corals (e.g., D'Elia and
Webb, 1977; Muscatine and D'Elia, 1978), with the large
reef-flat populations of holothurians being the only other
major population to be studied (Webb et al., 1977).

D'Elia and Webb (1977) studied dissolved nitrogen flux
in corals and focused primarily on rates of nitrate uptake.
Working with intact coral colonies in incubation chambers,
they found uptake to be light-dependent. Uptake was local-
ized in the coral tissue or its algal symbionts and did not
occur in the bare skeletons left when living coral tissue
with its contained zooxanthellae was removed. Uptake was
found to fit the active-transp>ort model of enzyme kinetics,
with a half-saturation constant of 249 ± 247 nM and a
maximum uptake rate of 5.69 ± 1.11 ng-atoms mg N~'
min~' (29.9 ± 7.1 ng-atoms N mg chl a~^ min~').
There appeared to be a threshold ambient nitrate concen-

REEF PROCESSES

167

tration (57 ± 47 nM) below which uptake did not occur.
A limited number of organic nitrogen determinations indi-
cated that a slight efflux of such comf)ounds occurred. A
model of nitrogen flux in Pocillopora speaes was produced
in which, at typical ambient levels, ammonium uptake
appeared to be about twice as great as nitrate uptake.
D'Elia and Webb concluded that, while dissolved nitrogen
taken by corals might be nutritionally important, it was not
likely that the organisms could sustain high growth rates
on this nitrogen source alone; hence, some ingestion of
particulate nitrogen was probably necessary.

In a later paper, Muscatine and D'Elia (1978) studied
ammonium flux in intact coral colonies held in incubation
chambers. Of several species tested, only those containing
zooxanthellae were found to take up and retain NH4 .
Uptake and retention were enhanced by light, and the
authors concluded that uptake during a normal daylight
period is sufficient to sustain NH4^ retention during the
night. The pattern of uptake kinetics for several species
indicated that a two-process mechanism might be involved.
When a diffusion correction was made, uptake kinetics
could be characterized by the Michaelis-Menton equation.
Corals without symbiotic algae were found to release NH4
during incubation experiments.

Webb et al. (1977) assessed the biomass and nutrient
flux in populations of the sea cucumber Holothuria atra
which are abundant on the windward reef flats of
Encwetak. They estimated average densities of three
animals m~^, with a median weight of 60 g, in the
zone of small heads on a coralgal transect. Size distribution
of individuals was found to be negatively correlated with
water current velocity.

Ammonium release by the population was equivalent
to 9% of total release by the reef community and was
weight-specific in H. atra, H. difficilis. and Actinopi;ga mau-
ritiana (I.e., the size-specific release rate was constant for
different-sized organisms). Urea was also released by
animals in incubation chambers. For H. difficilis, the total
release consisted of 57% ammonium, 17% urea, and 26%
unidentified nitrogen compounds in a 2-g animal. For all
three species, the ratio of N:P release was 42:1 for 60-g
animals and 25:1 for 1-g animals.

Webb et al. (1977) also analyzed reef-flat sediments
and the gut contents and fecal pellets of H. atra for total
and organic carbon. Of the total carbon in the sediments,
3% was estimated to be organic; 10% of the carbon in
fecal pellets was estimated to be organic. The authors
stated that this finding could be accounted for by the dis-
solution of CaCOs in the animals' guts, by selective feed-
ing, or by both of these processes. The sediments were
estimated to consist of 0.4% organic C and 12% inorganic
C by dry weight; fecal pellets were 1.3% organic C and
11.6% inorganic C. The dissolution of CaCOs by
holothurian populations was estimated to be approximately
2.5 g m d~\ equivalent to about 25% of the net calcifi-
cation rate for the reef flat as a whole. Webb et al. con-
cluded that H. atra is a selective feeder and that it ingests
euid egests materials considerably richer in organic carbon

than the general sediment; assimilation efficiency was
estimated to be approximately 40%.

Phosphorus Flux in Individual Populations

Studies of phosphorus flux in individual populations
started before those of nitrogen flux and have been more
numerous and concerned with a larger variety of organ-
isms. The first was reported by Pomeroy and Kuenzler
(1967), who examined phosphorus content and elimination
rates for dominant reef animals of several taxa; these two
measurements were then integrated to estimate the flux of
P through the organism.s, expressed as turnover times.
Herbivorous fishes appeared to receive just enough P for
growth and reproduction, as reflected in somewhat lower
excretion rates and longer turnover times than would be
predicted for organisms of their size by standard relation-
ships. In the important herbivorous fish Acanthurus (turn-
over time, 410 days), the ingestion of small quantities of
animal food, even if taken only incidentally to grazing on
algal filaments, might be an important phosphorus source.
Carnivorous animals and deposit feeders were calculated to
receive excess P in their diets, with turnover times (12 to
4000 days for a large size range) not significantly different
than they would be in ecosystems with a more abundant
supply of that element. Of five coral species, four showed
very long turnover times (10^ to 10^ days) and little P loss
to the environment. The giant clam Tridacna crocea
showed a typical turnover time (about 900 days) for a mol-
lusc of its size, and most of its P loss was that incor-
porated into living zooxanthellae which were subsequently
lost. In general, the phosphorus economy of animals con-
taining algal symbionts seemed to be quite varied. Zoo-
plankton had turnover times approximating 1 day.

Additional studies of the exchange of phosphorus
between organisms and reef waters were conducted by
Pomeroy et al. (1974). Although algal mats dominated by
the blue-green Schizothrix showed an active uptake of
radioactively labeled P04"^ in the light, they also showed a
continuous loss. Pomeroy et al. thought that the loss was
at high enough rates that, if a bell jar were placed over the
pavement community for a short time in the dark, changes
in dissolved phosphorus could be detected chemically in
the enclosed water mass. They also considered it possible
that as much as 50% of the photosynthetic products of the
pavement community might be released as soluble organics
containing no P. Algal mats dominated by the articulated
coralline red Jania showed no net uptake or loss of phos-
phorus when incubated. Algal-encrusted pebbles showed an
insignificant suggestion of uptake and no evidence of loss.
The corals Acropora and Heliopora showed no net uptake
or loss after equilibration, while Millepora showed continu-
ous uptake. Compartmental analysis of the former two spe-
cies suggested that there was a pool of phosphorus in the
coral tissue in equilibrium with P04^'^ in the ambient water
and isolated from the P demand of the zooxanthellae.
Pomeroy et al. cited Muscatine's earlier (1967) work as
evidence that much of the photosynthate released from the

166

MARSH

zooxanthellae contains no P; hence, tha coral-algal associa-
tion should require new P only for growth and replacement
of zooxanthellae. Corals may thus recycle phosphorus (and
nitrogen) so effectively that their requirements are met by
the solid food they ingest, with no uptake from ambient
water being necessary; this is possible only for a slow-
growing community.

With respect to transfers between trophic levels, it was
not clear to Pomeroy and co-workers which consumers
benefit from the coral production, although mucus may be
one of the major coral products (a recurrent theme which
will be discussed further in the next section). Phosphorus
in the algal filaments cropped by fish is probably recycled
largely to P04"^ by excretion. At least 75% of such P may
be recycled in 1 day in this kind of system. The authors
found significant recycling of P in all reef communities
examined. They concluded that the principal reef commu-
nities at Enew?tak are not limited by P and have evolved
either internal (biochemical) or external (food chain) recy-
cling loops to satisfy apparent P demands. These conclu-
sions were thus consistent with the ways that ecologists
have thought of reefs since publication of the Odutn and
Odum paper.

Arsenate uptake and reduction by the coral Pocilhpora
verrucosa are somewhat related to ohosphorus cycling and
were examined by Pilson (1974). In incubation expcri
ments, P. verrucosa was found to remove arsenate from
solution and convert some of it to arsenite. which reap
peared in the ambient seawater. This suggests that organ
isms other than bacteria may be responsible for maintain-
ing som.c of the arsenic in seawater in a reduced form.
Reduction of arsenate may be a mechanism to allow the
loss from the liviiig coral cells of arsenate incidentally
transported in along with phosphate.

D'Elia (1977) further examined the uptake and release
of dissolved phosphorus by corals in incubation exp>eri-
ments. He found that the net uptake of reactive phos-
phorus from seavater by coral? containing zooxanthellae
v/as not sufficient to offset simultaneous losses of organic
phosphorus; hence, there was a net loss of total P. A coral
without zooxanthellae was unable to remove net amounts
of reactive P from solution, evon at levels greater than the
normal ambient levels in reef waters. Reactive P uptake
was found to be light sensitive, was highly temperature
dependent, showed characteristics of Michaclis-Menton
enzyme kinetics, and could be inhibited by arsenate An
active-transport mechanism, thus appeared to be involved
in P uptake. The kinetics of net reactive P uptake were
described by a Michaelis-Menton equation modified to
include a correction for an efflux of reactive P going on at
the same time. The m.ean half-saturation constant was 377
nM and the mean maximum rate of uptake was 29.3 ng-
atoms P mg chl o ' h V DElia concluded that corals con-
taining symbiotic algae are ti.us unable to obtain all their P
requirements by means of reactive P uptake at typical
environmental concentrations but that their ability to obtain
part of the P requirements in this fa<:hion may help them
to flourish in water low in available P. This is probably

further enhanced by the presence of mechanisms for effi-
cient recycling of P within the symbiotic association.

Finally, Webb et al. (1977), in the paper previously dis-
cussed regarding nitrogen flux, briefly considered phos-
phorus along with nitrogen in the nutrient-flux measure-
ments of Holothuria atra populations They reported that
the release of P followed the general rules in the literature
for size-metabolism relationships.

TROPHIC TRANSFERS

Energy and nutrient transfer between trophic levels is
perhaps the area of energy and materials flux which has
been the least studied with respect to the amount of
research that is required for a comprehensive understand-
ing This is an area of great interest for ecologists and for
those concerned with the increasingly important area of
reef management A major effort would be required for
such a comprehensive understanding. As with so many of
the other topics discussed in this chapter, the first general
effort in this area was that of Odum and Odum (1955). In
addition to a description of trophic pathways and quantita-
tive trophic pyramids discussed earlier, they made some
preliminary attempts to quantify trophic transfers. While
their work pointed the way for future research, it was
hardly definitive; other researchers, so far, have been slow
to take up the challenge.

Efforts since the Odums' study have dire'"ted attention
to three main a'eas of trophic relationships. The greatest
interest, cutting across these areas, appears to have been
on transfers from the windward reef flats into the lagoon,
with much less attention directed to transfers between
trophic levels within the reef flat subsystem Aside from
the relatively small-scale effort by Webb et al. (1977) to
understand nutrition of the large populations of sea
cucumbers on the reef flats (discussed previously), the
greatest interest at Enewetak, as elsewhere, has been with
the feeding relationships of the diverse fish community.
Most of the interest has been either on the recurrent
theme of "pseudoplankton'" (relatively large algal fragments
broken off from seaward reef zones and carried lagoon-
ward) or on the role of coral mucus in trophic relation-
ships, particularly those of the lagoon community. The
mucus interest, at least, fits in with the general interest of
many marine ecologists in the role of detritus and particu-
larly of organic aggregates that first came to prominence in
the late 1960s and carried over into the 1970s. It would
be desirable to have a careful evaluation of the general
framew'>rk of trophic relationships which could then be
used to point out directions for future trophic research. A
new overview paper such as that of Odum and Odum is
needed because of recent understandings of marine ecosys-
tems in general and reef ecosystems in particular.

Trophic Rplationships of Fishes

A major early paper on trophic relationships of reef
fishes was that of Hiatt and Strasburg (1960). who exam-
ined the feeding habits of 233 species. They distinguished

REEF PROCESSES

169

seven major groups of fishes: algal feeders, detritus
feeders, scavengers, plankton feeders, carnivores, coral
polyp feeders, and omnivores. Their algal feeders were
divided into four categories: (1) those which subsist on uni-
cellular algae (mullets and some blennies); (2) grazers,
which crop very closely to substratum and ingest some of
it along with the algae (surgeonfishes, damselfishes, gobies,
blennies, triggerfishes); (3) browsers, which use cutting
teeth for biting off algal fronds or filaments above the sub-
stratum and do not take in any of the nonalgal material
(surgeonfishes, damselfishes, triggerfishes); and (4)
incidental algal feeders, which feed primarily on other
materials (butterflyfishes, wrasses, parrotfishes, gobies,
puffers). The detritus feeders are represented by relatively
few species (mullets, gobies, blennies). The scavengers
included only the nurse shark In addition to the manta
ray, the plankton feeders included some round herrings,
halfbeaks, silversides, damselfish, and wrasses. The car-
nivores included a large number and variety of fishes fall-
ing into five groups: (1) those feeding on fossorial fauna,
(2) those feeding on benthonic fauna, (3) those feeding on
mid-water fauna, (4) resident roving carnivores, and (5)
transient roving species. The coral polyp feeders included
browsers, grazers, and feeders on branching coral tips.
Both facultative and incidental omnivores were recognized.
Colin, in chapter 7 of this volume, considers the feeding
relationships of fishes in greater detail.

Hiatt and Strasburg (1960) raised several points of
interest in understanding trophic relationships of the fishes.
These points have perhaps not been pursued sufficiently in
subsequent research. They regarded benthic invertebrates
as the chief organisms for converting particulate and col-
loidal organic material to animal protoplasm, with the
energy and materials then reaching the fishes through
predators on the invertebrates. Hence, they stated that an
understanding of the primary producers and the trophic
relationships of the higher organisms (i.e., fishes) is rela-
tively well advanced in the reef ecosystem, "but there still
remains to be known the role of the myriads of inver-
tebrates which inhabit the reef."

Subsequent studies have continued to focus on the pri-
mary producer level and on the fishes, and the role of
invertebrates is still relatively unknown. Hiatt and Stras-
burg pointed out that herbivorous species dominate the
fish fauna of Marshallese reefs; this is a generalization
which apparently applies to most reefs. Moreover, "It is
still an enigma why the biomass of herbivorous fishes is so
proportionately great on tropical reefs where the large
seaweeds are not abundantly available, and so propor-
tionately small along coastal shores in the temperate and
arctic seas." This may be less an enigma than Hiatt and
Strasburg thought, considering that algal productivity is
high in reef ecosystems and that the rate of energy
transfer through these producers can be high even with a
small biomass. It is probable, however, that the role of
algal turfs has been insufficiently appreciated and that
these multispecies assemblages, which are visually
unimpressive but are apparently subjected to heavy grazing

pressure, have high rates of productivity even with their
low-standing crops, (See Marsh, 1976, for further develop-
ment of this point.)

Hiatt and Strasburg further pointed out that, among
the fish fauna, surgeonfishes are the most important group
in converting primary productivity into animal tissue; they
convert large amounts of energy, whether or not they are
efficient. This impression has generally persisted among
reef ecologists. Hiatt and Strasburg also reported that all
parrotfishes they examined had scraped coral polyps and
that these animals may scrape smooth, algal-covered rocks
as well. This was disputed by some later studies, which
emphasized the grazing role of parrotfish rather than their
ingestion of coral polyps, although the latter activity is gen-
erally acknowledged to occur to a greater or lesser extent.
The Hiatt and Strasburg study has continued to be influen-
tial in shaping our thinking about reef ecosystems.

Hobson and Chess (1978) studied trophic relationships
among fishes and plankton in the nearshore lagoon adja-
cent to windward islets. They found that feeding patterns
differed sharply between day and night and were strongly
influenced by current patterns. The adults of most diurnal
planktivorous fishes were numerous in certain places
where tidal currents were strong but much less numerous
where such currents were consistently weak. Strong-
current areas are rich and weak currents are poor in the
major zooplankton prey of the fishes (e.g., copepods,
larvaceans, and fish eggs). On the other hand,
zooplankton-poor waters close to island lees and interisland
lees are rich in reef debris; those fishes that could subsist
in those areas were abundant. Dascy/lus reticulatus was
numerous in such environments, though less so than where
currents were strong, and took algal fragments as an
important but secondary part of its diet. Other species that
fed largely on algal diets could be equally abundant in
strong- and weak-current areas or more numerous where
currents were weak.

Major nocturnal planktivores, in contrast to the diurnal
feeders, were concentrated where the currents were weak
but were relatively sparse where the currents were strong.
These were found to be strictly carnivores that prey mostly
on large zooplankers (large calanoid copepods, mysids, iso-
pods, gammarid amphipods, postlarval carideans, and
brachyuran megalops) which were absent in the nearshore
water column by day. Such prey were reported to gener-
ally find conditions unfavorable where strong currents flow.
Most of these found shelter on or near specific nearshore
substrata during the day and entered the water column
only at night; others were found to be in deeper offshore
waters by day and moved inshore at night.

Overall, the pattern was clear to Hobson end Chess;
most diurnal fish favored zooplankton rather than algal
fragments. This pattern was somewhat at variance with the
impressions that previous researchers had formed. The fish
morphology that permits faster swimming is more
developed in planktivores that swim farther from the reef;
Hobson and Chess interpreted this as an adaptation to
escape predators.

170

MARSH

Smith and Paulson (1974) studied four transit times
and gut pH's in two parrotfish species, Scarus jonesii and
S. gibbus. They found that both species begin feeding
around first light (reported to be about 07:30 hr) and con-
tinued feeding until about 19:00 hr, at which time indivi-
duals began aggregating in groups of 15 to 50 and then
dispersed at last light (about 19:50 hr). They were seen
feeding only on dead coral with its covering of filamentous
algae, contrary to reports of Hiatt and Strasburg (1960)
that these species feed on live coral. Smith and Paulson
concluded that material ingested by the parrotfish at dawn
is often evacuated within 4 hr and that food consumed at
dusk passes through the gut in 6 hr or less. Such calcu-
lated transit times of 4 to 6 hr correspond to filling the gut
at least twice a day. Feeding is intermittent.

In feeding S. jonesii, the anterior three gut regions
(pyloric caecum, small intestine, large intestine) were found
to be more acidic than seawater, with values of 6.8 to 7.5;
the rectum (pH = 8.2) was not. In S. gibbus, all four
regions of the gut (pH ranging from 6.4 to 7.5) are more
acidic than seawater. Smith and Paulson emphasized that
these were feeding fishes rather than those with empty
guts. They concluded that CaCOs mai; dissolve in the par-
rotfish gut.

Reese (1977) considered the coevolution of corals and
coral-feeding fishes of the family Chaetodontidae. He
placed these buttcrflyfishes into one of three feeding
categories: coral feeders, omnivores which feed on bcnthic
invertebrates other than corals, and plankton feeders. The
coral feeders may be obligative or facultative. At
Encwetak, 10 of the 17 species studied were coral feeders
(with four of these being obligate coral feeders), five were
omnivores, and two were planktivores. Laboratory studies
conducted in Hawaii showed that Chaetodon thfasciatus
and C ornatissimus preferred the coral Pocillopora dam-
icornis over Montipora verrucosa over Porites compressa.

Nolan et al. (1975) examined the fish communities
inhabiting two small nuclear test craters at Enewetak. They
found the standing crops of herbivorous and carnivorous
fishes to be 35.7 and 61.3 g m~ , respectively, in
LaCrosse Crater and 5.7 and 16.8 g m~^, respectively,
in Cactus Crater. This was higher than the 10.3 and
4.6 g m^ , respectively, reported by Odum and Odum
(1955) for their ^one of large heads. In the two nuclear
test craters, carnivores constituted 74.7% and 63.2% of
the total biomass, but Odum and Odum reported
herbivorous fish biomass to be four to five times that of
carnivores. Nolan et al. estimated that 100 kg of goatfish
alone might be harvested from the two craters every 1 to
2 days because there is continuous immigration.

The Role of Detritus

The role of detritus as a link between the reef flat and
the lagoon was first noted by Odum and Odum (1955),
who observed the transport of algal fragments from the
back-reef zones into the lagoon. The next paper was by
Marshall (1965), who believed that particulate matter car-
ried off the windward reefs might constitute a substantial

contribution to the trophic system within the lagoon. He
stated that most detritus on cleared filters, sampled from
water crossing the reef flat, appeared to be of plant origin,
but he also noted the presence of amorphous organic
aggregates. A lagoon sample from a coral knoll appeared
to be similar to that of the reef flat. He found more
detritus over the reefs and in the lagoon than in samples
from the deep pass or seaward of the reef front. His
values for combustible material trapped on glass filters
were at least an order of magnitude greater than those of
the earlier Odum and Odum study and sometimes almost
two orders of magnitude greater. Ash-free dry weights
from the lagoon averaged more than 0.1 g m . Chloro-
phyll a values ranged from 0.08 to 0.14 mg m""' in the
channels, 0.21 to 0.33 on the coral-algal ridge, 0.15 to
0.39 in waters crossing the reef flats, and 0.16 to 0.61 in
the lagoon. Ash-free dry weights were 0.04 to 0.15 g
m ■' in the channels, 0.10 to 0.99 on the coral-algal
ridge, 0.15 to 0.62 on the reef flats, and 0.06 to 0.22 in
the lagoon.

Johannes (1967), in the first paper to focus on the role
of coral mucus, further considered the ecology of organic
aggregates and noted that these showed a markedly
increased concentration as oceanic water crossed the reef
and entered the lagoon. These aggregates consisted largely
of coral mucus. Johannes estimated the export of mucus
into the lagoon as 20 mg m"'' h~', or about 20% of the
total reef production and 40% of total coral respiration. A
few meters lagoonward of the drop-off at the back of the
reef flat, organic aggregates were usually the only identifi-
able suspended objects in the water column; most of the
algal fragments and sediment particles had settled out. In
laboratory experiments, Artemia nauplii survived longer
and grew faster in water with added mucus than in filtered
seawater.

Coles and Strathman (1973) made further observations
on coral mucus "floes" and their potential trophic signifi-
cance. They found that visible mucus floes contain signifi-
cant quantities of organic matter compared to microscopic
suspended particle concentrations in surrounding water.
Carbon to nitrogen ratios suggested that suspended mucus
floes are enriched with nitrogen compared to more recently
secreted coral mucus or microscopic particulate organic
matter. Freshly collected mucus, after drying, had organic
contents comparable to other biological materials, 26% C
and 3% N; caloric values were 3.95 gcal mg~' (ash-free
dry weight) for mucus collected from Fungia scutaria.
Suspended mucus floes collected on the lagoon side of the
windward reef at Enewetak closely resembled mucus
obtained from Acropora in the laboratory and contained
algae, occasional protozoa, organic debris, and inorganics.
Mucus floes from different corals differed in C:N ratios and
in the total quantities of organic C and N.

Gerber and Marshall (1974a, 1974b) considered the
role of reef pseudoplankton in trophic systems of the
lagoon. Gut analyses of Undinula vulgaris (a calanoid
copepod), Oikopleura longicaudata (a larvacean), and
several species of planktivorous fishes showec. that detritus

REEF PROCESSES

171

amounted to 95% and 85%, respectively, of gut contents
of the first two animals; chlorophyll was present in 2% and
6%, respectively, of the food mass. Planktivorous fishes
were reported to consume, in addition to zooplankion, a
substantial amount of detrital algal fragments. Fragments
of the nitrogen-fixing blue-green algae Calothrix were abun-
dant in the gut samples and were taken to be a primary
contributor, both directly and indirectly, to lower C:N
ratios in the lagoon. Chlorophyll a and phaeopigment lev-
els in the lagoon waters were small, amounting to 0.098
mg m"^ and 0.085 mg m~^, compared to total particulate
C values of 20.5 mg m"'^. Waters over and behind the
reef had lower levels of particulate C and N and higher
C:N ratios than incoming oceanic water. Gerber and
Marshall concluded that there is substantial input from the
reef to the lagoon and that this input probably supports a
more abundant zooplankton in the lagoon than would oth-
erwise be the case. Gerber and Marshall stated that
decomposition of algal fragments by fish digestion may be
the first step in its transformation to a form that can be
consumed by zooplankton.

In another study, Johannes and Gerber (1974) exam-
ined the import and export of net plankton by a portion
of the reef-flat community by placing plankton nets
(60-M mesh) immediately upstream and downstream of the
coral zones. Differences between these upstream and
downstream nets showed a net import of organic C, N, P,
benthic algal fragments, fecal pellets, and zooplankton by
the coral zones. (However, analyses of small-volume water
samples showed a net export of particulate C and particu-
late N for the who|p transect.) Benthic algal fragments
outweighed all other imported components combined; fecal
pellets accounted for the rest of the detritus. Most algal
fragments consisted of the red Asparagopsis or the blue-
green Calothrix. Johannes and Gerber calculated that sev-
eral thousand meroplankters were exported and several
thousand holo- and meroplankters were imported daily by
each square meter of reef surface. Some removal of algal
fragments was due to settling out rather than feeding by
animals. Johannes and Gerber concluded that reef commu-
nities are efficient traps of net plankton and that this may
contribute to downstream changes in community composi-
tion and possibly to the limited width of interisland reef
systems (through downstream plankton depletion).

Marshall et al. (1975) made additional observations on
particulate and dissolved organic matter in reef waters.
They reported that high concentrations of particulate
organic carbon occur in the environs of reefs and may be
attributed to the reef community itself. Changes in dis-
solved organic carbon (DOC) concentrations of waters flow-
ing across reef flats are relatively small and inconsistent.
The lack of distinct net increases in particulate organic
matter (POM) of waters flowing across shallow reefs sug-
gests that some of the released particles may be entrapped
and consumed by the community. The composition of par-
ticulate matter is extremely varied, but there is always
very little phytoplankton. As in most waters, particulate
organic carbon (POC) levels are an order of magnitude less

than DOC levels. Marshall et al. stated that there is an
impressive increase of POC on rises and reef crests for all
reefs studied, and there is a decrease in the ratio of
DOC: POC from open ocean waters across reefs and into
lagoons (changing from 103 to 33 for material trapp>ed on
glass filters).

The role of detritus in trophic relationships within the
lagoon is considered further in Chapter 10 in this volume.
The reader is referred to that chapter for a discussion of
the magnitude and importance of inputs of detritus from
the reef flats to the lagoon and for an integration of the
various subsystems of the total atoll ecosystem.

ENEWETAK RESEARCH
IN PERSPECTIVE

Although much pioneering work was done at Enewetak
over the years, interest has now shifted to a number of
other localities because of the shutdown of the Enewetak
facility, the establishment of facilities elsewhere, and the
rapid growth of reef research in the last decade. It is
worthwhile to consider how the earlier Enewetak work fits
into the broader context of more recent knowledge.

Advances at the community/ecosystem level have
come in considering whole-atoll systems rather than simply
reef flats, in examining temporal and spatial variations
rather than relying on a restricted set of observations, and
in more fully integrating nutrient fluxes into the total meta-
bolic picture. Recent work is probably more striking for
confirming and amplifying insights derived from Enewetak
research than for reversing any major conclusions reached
at that level.

Kinsey (1979, 1983) has considered "standards of per-
formance" by reef ecosystems with respect to primary pro-
duction and carbon turnover. He summarized metabolic
studies and emphasized the considerable uniformity
reported for reef flats from different latitudes and with
differing biological makeup. The mean gross productivity
for 16 studies was calculated tobe7.9gCm~ d (stan-
dard deviation [S.D.] is 2.7), with a mean calculated 24-h
respiration also of 7.9 g C m~^ d"' (S.D. is 5.0). These
are general values which are strikingly similar to the
Enewetak-only values discussed earlier in this chapter.
Further striking confirmation of the earlier Enewetak work
comes from Kinsey's summary of P:R values consistently
approximating 1.0 and his summary indicating that plank-
ton metabolism is at least an order of magnitude lower
than activity of the ecosystem as a whole. Furthermore,
the Smith and Kinsey (1976) suggestion, based partly on
earlier Enewetak work, of a bimodal model for calcification
rates also appears to have been borne out by subsequent
work at other localities. Hence, Kinsey (1983) suggested a
generalized bimodal picture of (1) reef flats and all exten-
sive, present-day metabolically active perimeter zones
(whether an outer reef crest or the windward edge of a
lagoon ward patch reef) with a gross P of 5 to lOgCm
d~ and a calcification rate (G) of 3 to 5 kg CaCOa m~
yr^^, and (2) sand/rubble areas with contrasting rates of

172

MARSH

1 g C m~^ d~^ and 0.5 kg CaCO^ m~^ y~\ respectively.
The first mode is itself regarded as a composite and can
be further subdivided. In particular, it may include areas of
continuous coralgal cover and discrete heads with
P = 20 and G = 10, where water depth and circula-
tion are adequate. Kinsey proposed a "standard" reef flat
with a gross P of 7 ± 1 g C m~^ d"\ a P:R ratio of
1 ± 0.1, and a G of 4 ± 1 kg CaCOs m"^ yr"\
where "reef flat" conforms to the concept of a fully
developed (at or near present-day sea level), areally exten-
sive (at least 100 m across), high-activity zone of the
coralgal type.

Kinsey (1982) also considered comparative aspects of
calcification rates and reef growth (accretion) between
Pacific and Caribbean reefs and attempted to resolve what
seemed to be discrepancies between the two oceans. He
noted that the apparently higher rates in the Caribbean
were based primarily on long-term accretion rates deter-
mined from stratigraphic methods and that most estimates
in the Pacific were derived from short-term chemical
changes in resident water masses. He concluded that there
probably was a faster growth of Caribbean reefs during the
Holocenc epoch, with major factors being differences in
sea level, tectonics, and wave energy. The particular com-
bination of these factors in the Caribbean led to diminutive
surface features and proportionately greater seaward
slopes there, with wider expanses of reef flat and propor-
tionately smaller seaward-slope areas in the Pacific. Kinsey
further concluded that any interocean differences in the
calcifying capacity of reef communities are small. Hence,
additional research has served to put earlier Enewetak
work into a broader context but has not drastically altered
earlier conclusions resulting from Enewetak work.

One earlier impression probably arising from Enewetak
research (e.g., Odum and Odum, 1955) should be modi-
fied. As pointed out by both Smith (1983) and Kinsey
(1983), initial reports of high productivity of reef-flat com-
munities led to the tendency to regard whole-reef systems
or "coral reefs" as being one of the world's most produc-
tive ecosystems. However, if the complete system, particu-
larly including the lagoon and extensive sand/rubble areas,
is considered, the rates of production are much more mod-
est. Kinsey (1983) tabulated published values of commu-
nity metabolism for complete reef ecosystems and showed
P (gross) ranging from 2.3 to 6.0 g C m~^ d~\G ranging
from 0.5 to 1.8 kg CaCO m"^ yr^', and P:R ratios hold-
ing constant at 1 for the five studies. The distinction
between particular reef communities and whole reef
ecosystems is one which must be more carefully drawn in
future studies.

Somewhat related to this point is the question of the
metabolism of sediment communities, which comprise a
substantial proportion of the whole system. Harrison
(1983) studied this question by placing plastic domes over
such communities at Enewetak and monitoring O2 and
CO2 fluxes. He derived empirical respiratory quotient
values of 0.8 and repwrted that more carbon is respired by
the sediment community than is produced. He calculated

that excess production exported from the windward reef
flats was sufficient to support the metabolsim of these dis-
tinctly heterotrophic sediment communities. Both produc-
tion and respiration showed a decline with depth. Accord-
ing to Harrison, "Biotic and functional comparisons
between Enewetak and Kaneohe Bay, Hawaii, suggest
metabolic and structural similarities between these physio-
graphically disparate coral reef ecosystems." Thus, there is
a recurring theme of similarities between reef processes at
Enewetak and those in other seemingly different reef
ecosystems.

Several important insights about nutrient availability
had their origins at Enewetak but have been sharpened
and extended by work at other localities. After the initial
measurements of nitrogen fixation at Enewetak (Webb et
al., 1975; Wiebe et al., 1975), there followed a number of
other studies of this process at other localities (e.g., Cross-
land and Barnes, 1976; Burris, 1976; Capone et al.,
1977). However, Szmant-Froelich (1983) pointed out that
such measurements have generally been restricted to reef
flats or back-reef areas and that denitrification (conversion
of N03^ to N2) has not been adequately measured in any
reef environments.

Entsch et al. (1983), working on the Great Barrier
Reef, also conducted research on nutrient availability. They
found a large pool of nitrogen and phosphorus in car-
bonate sediments and in the interstitial waters of the sur-
face layers of sediments. Nutrient concentrations were con-
sidered to be sufficient to allow high rates of uptake by
epilithic algae. This is apparently an imfxsrtant recycling
mechanism in reef systems.

Andrews and Muller (1983), building upxDn DiSalvo's
(1971, 1974) idea of regenerative spaces, measured
nutrients in a lagoonal patch reef of the Great Barrier Reef
complex and studied rates of water percolation through
the reef. Concentrations of NOa" and P04~"' in cavities on
the vertical face of the reef were found to be significantly
higher than in the surrounding water. Nitrogen export
through tidal flushing of their patch reef was reported to
be of the same order of magnitude as export from the
Enewetak reef flat studied by Webb et al. (1975); this
export was presumably supported by nitrogen fixation. The
molar ratio of nutrient regeneration rates was calculated to
be 140:2:7 for NO3 :N02":P04"^; if NOs* and NO2" were
lumped, the N:P regeneration ratio approximated 20:1.

Smith (1983) further sharpened our understanding of
productivity and nutrient relationships in reef ecosystems
by pointing out that the net productivity of the whole-reef
system (rather than simply the reef-flat portion of the
system) is low. He estimated it to be less than 100 mgC
m~^ d~\ or within about the same range as "new" pro-
duction in open-ocean planktonic systems. As he and
Szmant-Froelich (1983) pointed out, any net production in
the ecosystem as a whole requires an input of new
nutrients. Recycling of nutrients already in the system can
support high gross production if the P:R ratio is exactly 1
and if the recycling is efficient. If recycling processes are
not efficient then there must be an input of new nutrients

REEF PROCESSES

173

to maintain the steady-state system. With low net produc-
tivity and efficient recycling, whole reef ecosystems should
not be expected to require large inputs of new nutrients.
Hence, the apparent paradox of high-productivity reef sys-
tems in the midst of nutrient-poor waters, as perceived in
the earlier studies, can now be viewed as not too surpris-
ing. Smith further argued that, since reef production is
dominated by benthic plants with a C:N;P ratio of approxi-
mately 550:30:1 (Atkinson and Smith, 1983), they can
produce more net carbon per unit of nitrogen and phos-
phorus availability than can planktonic systems with a
C:N:P ratio of 106:16:1.

Atkinson (1981, 1982) challenged earlier ideas regard-
ing the cycling of phosphorus in reef metabolism and
argued that there is not a tight cycling of that element for
a whole reef flat. However, because of a high advective
flux of phosphate over most reef flats, the system can
depend primarily on exchange with the water column for
its nutrients; and only 10% of the P04~'^ available to reef
producers might be recycled through the water column.
This challenge to earlier ideas has contributed to a continu-
ing interest in the comparative roles of nitrogen and phos-
phorus fluxes in reef systems and the question of which of
these elements is limiting to metabolism. For example, in a
study of the whole lagoonal system at Christmas Island
(Kiribati), Smith et al. (1984) argued that net metabolism
of the system is limited by the availability of phosphorus.
Ideas are developing rapidly, and additional work is likely
to be reported in the near future. Crossland (1983)
recently provided an overview of nutrients in coral reef
waters.

Studies of individual populations of reef organisms
have taken place at a larger number of geographic loca-
tions than have ecosystem studies. Work at Enewetak has
not played the fundamental role for the former type of
study that it has for the latter. Literature on the biology
and ecology of individual populations is extensive and
diverse. The reader is referred to the recent paper by Lar-
kum (1983) for entry into the literature on productivity of
plant populations, to the paper by Chalker (1983) for
recent calcification studies of corals and other animals, and
to the papers by Muscatine (1983) and Chalker and Dun-
lap (1983) for work on metabolism and production of
corals.

SUMMARY

A number of significant single-investigator and team
studies have been conducted at Enewetak and have
contributed to a general understanding of reef ecosystems
and to the development of methodology for studying such
systems. Many of these studies have focused on the reef
flats as the metabolically dominant subsystem of the whole
atoll. The early study by Odum and Odum (1955), which
attempted to relate structure and function in the windward
reef-flat community, established a conceptual framework
that is influential even today. Other important studies have
focused on community metabolism, calcification processes

at the ecosystem and organismal level, nitrogen and
phosphorus cycling on the reef flats and in individual
organisms, the trophic role of detritus, nutritional sources
for corals, and the ecological relationships of reef fishes.

Table 1 summarizes most of the studies discussed in
this chapter. Some of the highlights are reiterated in the
following paragraphs.

Biological zonation is an important aspect of the struc-
ture of reef flats. Rates of community metabolism on such
flats are high, with gross productivities of 6 to 10 g C m~^
d~^ for coralgal communities and 12 g C m"^ d~' for
algal-dominated communities; 24-h ratios of gross produc-
tivity to respiration approximate or exceed 1.0. Rates of
calcium carbonate production on reef flats are also high,
approximately 4 kg m" yr~', with little apparent differ-
ence between day and night; corals are a relatively minor
contributor to the process at the level of the whole ecosys-
tem. Rates of nitrogen fixation (and nitrogen export) on
reef flats are again high, up to 1000 kg ha~^ y~\ and
help account for the high productivity. Important nitrogen-
fixing organisms include blue-green algae and bacteria.
Studies at Enewetak suggest that internal phosphorus
cycling within reef-flat communities is very efficient, with
little exchange between the benthic biota and the water
column; however, this conclusion has recently been chal-
lenged. The role of regenerative spaces has been examined
in an exploratory way; such internal reef spaces probably
deserve much more attention than they have received.

Various aspects of the biology of individual populations
have also been examined at Enewetak. The primary pro-
ductivity of the few algal populations that have been stud-
ied on a preliminary basis has been estimated at up to 2.3
gC m^ d for various species of Halimeda, with some-
what lower values for intertidal algal mats and calcareous
red algae. The productivity of filamentous boring algae,
originally thought by the Odums to be of major impor-
tance, is insignificant. Work conducted at Enewetak was
the first to demonstrate that zooxanthellae from the tissues
of corals and giant clams could release significant amounts
of radioactively labeled photosynthate, which could
presumably contribute to host nutrition. Other studies
investigating the oxygen balance of corals and their
enclosed zooxanthellae have indicated that at least some
species have the capability of obtaining all their nutritional
requirements from their symbiotic algae and that there can
be sun and shade differences in the same species growing
at different depths. Studies of the physiology of coral
calcification have demonstrated light enhancement of this
process (mediated by zooxanthellae), suggested the impor-
tance of active transport of calcium ions rather than diffu-
sion, and found a marked temperature effect on the
uptake of Ca. Cyclic variations in the radial density (den-
sity banding) of coral skeletons were calibrated by examin-
ing the distributions of radionuclides in the skeletons, then
used to estimate growth rates of individual colonies and
later found to result from seasonal temperature differences.
Nitrogen uptake in corals has been found to fit the active-
transport model of enzyme kinetics for at least some forms

174

MARSH

TABLE 1

Reef System Structural and Functional Characteristics
That Have Been Measured at Enewetak

Characteristic

Quantitative
estimate

Reference

Community net P (12-h day)

Coralgal transect

48gCm-^

Coralgal transect

3.0 9 Cm^^

Algal transect

8.6gCm-=^

Community gross P

Coralgal transect

lOgCm^^d"'

Coralgal transect

6gCm"2d"'

Algal transect

12gCm-'d-'

Gross PR ratio (24-h)

Coralgal transect

1.0

Coralgal transect

1.0

Algal transect

1.9

Organic CO2 flux

CaCOa precipitation

CaCOs precipitation

Vertical reef accretion

Nitrogen export from reef flat

Mucus exfxirt from reef flat
to lagoon

BOD, "internal sediments"

Net P, algal mats

Net P, Halimeda

Net P, calcareous red algae

Gross P, calcareous red algae

Pavement O2 evolution

Pavement P:R ratio (24h)

Photosynthate of pavement
community released as
soluble organics

Nitrogen fixation, algal-colored
pavement
Upper intertidai
Reef flat

Ratio of nitrogen :pfiosphorus
release in sea cucumbers

±0.2 to 6 moles m"^d"'
4 kg m~^ yr~'

-0.02 to 02 moles m"^d '
3 to 5 mm yr~'

1000kgha~V'
20 mg m"2fi"'

0.06 to 0.50 mgg~'h~'
0.6 to 2 ISgCm'^d^'
Up to 2 3 gCm^^d"'

0.66 gCm"^d"'

1.5 gCm-^d"'

Up to 5.5 X 10"^ ml cm"^h'

Up to 1.6

50%

34 X 10^' moles cm"^h '
55 X 10"' moles cm"^h"'

60-g animals

42:1

1-g animals

25:1

)issolution of CaC03 by

2.5gm-2d

holothurian populations

':R ratio (24-h), individual

coral colonies

Odum and Odum, 1955
Smith and Marsh, 1973
Smith and Marsh, 1973

Smith, 1974

Smith and Kinsey, 1976

Smith, 1974

Smith and Kinsey, 1976

Webb et al., 1975

Johannes, 1967

DiSalvo, 1971

Bakus, 1967

Hillis-Colinvaux,
1974

Marsh, 1970

Wells et al., 1973.

Wells et al., 1973

Pomeroy et al., 1974

Wiebe et al., 1975

Webb et al., 1977

Wethey and Porter, 1976b
(This table continued on next page.)

REEF PROCESSES

175

TABLE 1 (cont'd)

Characteristic

Quantitative
estimate

Reference

Shallow-growing
(sunny and overcast days)

Deep-growing
(sunny and overcast days)

Gross P required for R, individual
coral colonies

Kinetic parameters
Light response in coral
Pavora praetorta •

Maximum rate, gross P (Pm^x)
Light level for half-saturated
net P (K„)

NO 3~ uptake in coral PociUopora

Maximum uptake rate (V„^)
Half-saturation constant (KJ
Threshold \NOf] for
uptake to occur

NH 4" uptake in corals

(several spp.)
Diffusion
Active transport

' max

Reactive phosphorus uptake
in corals
V

Primary producer biomass
Primary consumer biomass
Carnivore biomass
Herbivore:producer ratio
Camtvore:herbivore ratio

1 79, 1.44

1.81, 1.50

Wethey and Porter, 1976b

Shallow-growing
(sunny and overcast days)

31%, 45%

Deep-growing
(sunny and overcast days)

30%, 42%

Linear growth rates, massive
coral colonies

Fauia pallida
Goniastrea retiformis
Porites lutea

5.7 mm yr~'

5.8 mm yr~'
7.6 mm yr~

Mass growth rates, massive
coral colonies
Fauia pallida
Goniastrea retiformis
Porites lutea

0.82 g cm %r"'
1.16 g cm~^yr~'
1.07 g cm'^yr"'

Highsmith, 1979

Wethey and Porter, 1976b

8.74 to 12.25 mg O2 • mg chl a ' h '
0.26 to 0.63 Em"^h'

D'Elia and Webb, 1977

5.69 ngatoms • mg-atom N ' min '
249 ngatoms 1"'
57 ng-atoms F'

Muscatine and D'Elia, 1978

0.52 to 0.93 1 • mgchla"'h"'

1.64 to 5.26 fimol • mg chl a"' h^'
0.29 to 1.05 Mgatoms 1"'

293 ngatoms • mg chl a ' h
377 ngatoms P'

703 g dry wt m"^
132 g dry wt m"^
11 g dry wt m"^
18.9%
8.3%

D'Elia, 1977

Odum and Odum, 1955
Odum and Odum, 1955
Odum and Odum, 1955
Odum and Odum, 1955
Odum and Odum, 1955

(This table continued on next page.)

176

MARSH

TABLE 1 (cont'd)

Characteristic

Quantitative
estimate

Reference

Standing crop of herbivorous
fishes in two nuclear
test craters

Standing crop of carnivorous
fishes in two nuclear
test craters

Bacterial standing crops
Sediments

Water

Standing crop of sea
cucumbers (Hohthuria atra)
in zone of small heads

pH of reef flat waters

Dissolved O2 in reef
flat waters

Alkalinity of reef flat

NH3 in reef flat waters

NO 3" in reef flat waters

Dissolved organic nitrogen
in reef flat waters

Particulate organic nitrogen
in reef flat waters

Reactive phosphorus in
reef flat waters

Organic phosphorus in
reef flat waters

Dissolved organic carbon
in reef flat waters

Particulate organic carbon
in reef flat waters

Suspended chlorophyll a
in water column
Channels
Coralgal ridge
Reef flats
Lagoon

Ash-free dry wt suspended
in water column
Channels
Coralgal ridge
Reef flats
Lagoon

35.7 and 5.7 g m

6L3 and 16.8 g m

10* to lO^cells •
g dry wt~'
80 to 200 cells ml"'

3 individuals m~^
60 g wet wt m~^

8.27 to 8.32

6.18 to 7.38 mgl"'

2.285 to 2.295 meq l'
240 to 287 nmol 1"'
109 to 169 nmol P'
1720 to 2145 nmol r'

157 to 210 nmol r'

169 to 174 nmol 1"'

152 to 155 nmoir'

1210 to 1230 n3 r'

24 to 26 M9 r'

0.08 to 0.14 mg m"^
0.21 to 0.33 mg m"^
0.15 to 0.39 mg m"^
0.16 to 0.61 mg m"^

0.04 to 0.15 g m"^
0.10 to 0.99 gm~3
0.15 to 0.62 gm"3
0.06 to 0.22 g m~^

Nolan et al., 1975
Nolan et al., 1975

DiSalvo, 1969
Johannes et al., 1972
Webb et al., 1977

Johannes et al , 1972
Johannes et al., 1972

Johannes et al., 1972
Johannes et al., 1972
Johannes et al., 1972
Johannes et al., 1972

Johannes et al., 1972

Johannes et al., 1972
Johannes et al., 1972
Johannes et al., 1972
Johannes et al., 1972
Marshall, 1965

Marshall, 1965

(This table continued on next page.)

REEF PROCESSES

TABLE 1 (cont'd)

Characteristic

Quantitative
estimate

Reference

CarbonnitrogBn ratio
Offshore waters
Lagoon waters

15:1
6.6:1

Webbetal., 1975

Composition of coral mucus
Carbon
Nitrogen
Caloric value

26%

3.95

gcal mg"' (ashfree dry wt)

Coles and Strathman, 1973

Organic carbon in sediments,
as % of total C

Webb et al., 1977

Organic carbon in fecal pellets

10%

Webb et al., 1977

177

of sea cucumbers, as % of
total C

pH of parrotfish guts

6.4 to 7.5

Smith and Paulson, 1974

of that element, with the symbiotic algae implicated as the
major agents in this update. Research on nitrogen cycling
through reef-flat populations of sea cucumbers was
reported to be a significant portion of total release by the
whole community. Exchange of phosphorus between reef-
flat organisms and overlying waters has been studied by
several workers, leading to the general conclusion that
exchange rates are slow and that there is a tight internal
cycling in most (but not all) cases.

There have been a number of studies of trophic rela-
tionships and transfers, although no subsequent study has
been as broad as that of Odum and Odum (1955). A
number of fish groups, in particular, have been dis-
tinguished at various trophic levels. The role of detritus,
both coral mucus and algal fragments, exported from the
reef flats into the lagoon has been emphasized but not
completely worked out, particularly with regard to its
importance for various fish populations.

Overall, there have been a large number of studies of
energy and materials flux at Enewetak, many of them of a
pioneering nature which pointed the way to a general
understanding of reef processes. However, ihe dominant
theme must be the preliminary nature of what has been
accomplished; a truly integrative understanding of reef
processes probably lies a long way in the future.

ACKNOWLEDGMENTS

I thank S. V. Smith, R. E. Johannes, and N. Marshall
for their reviews of the paper, although they may not
necessarily agree with everything in it. This is Contribution
No. 227 from the University of Guam Marine Laboratory.

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Chapter 10

Trophic Relationships in Enewetak Atoll

NELSON MARSHALL* and RAY P. GERBERf

'Universify of Rhode Island. Kingston. Rhode Island

02882: current address is P. O. Box 1056,

St. Michaels. Mari^land 21663

fSt. Joseph's College. North Windham. Maine 04062

INTRODUCTION

Some of the biologists who were attracted to Enewetak
Atoll after the marine research laboratory opened have
been studying species that are typical of reef environs and
plentiful in this setting. Some have been interested in eco-
logical features, particularly those of the well-developed
windward reefs; and some, who have focused on the reef
areas as an ecological subsystem, have been interested in
the processes of the atoll as a whole.

We start by noting three contrasting environments in
this large, but typical, atoll. First, there are the coral reefs
and knolls, the former almost completely enclosing the
atoll, the latter scattered through the lagoon and number-
ing over 2000. Then there are the of)en waters of the
lagoon. Finally, there is the lagoon benthic environment
(other than the coral knolls). In a real sense, and in com-
parison with the rest, the reefs and knolls are very produc-
tive, even though the oceanic waters surrounding the atoll
are low in nutrients and organic food sources. The level of
this productivity and the explanation (that the reef com-
munity dynamics involve rapid recycling rather than an
enrichment from seawater), are discussed in Chapter 9,
this volume, also in Odum and Odum (1955), and
Johannes et al. (1972). In contrast it would seem that the
lagoon waters are not productive; in fact, being extremely
clear (often one can see the bottom to depths of 50 m and
more), they give the appearance of being rather impover-
ished.

A Trophic Link Between the Reef
and the Lagoon

Rather spectacular populations of fishes are evident in
this dual setting of an impoverished lagoon and a produc-
tive, recycling coral reef seemingly low in net yield. This
raises basic questions as to the food dynamics involved:

how does the relatively closed reef system, together with
the oligotrophic lagoon, support such consumer popula-
tions? Part of the answer is because the reef areas are not
tight, unyielding environments as early studies suggested,
and because there is an impressive flow of detritus, mucus
flakes, algal fragments, and aggregated organic matter off
the reef.

When the flow of matter off the reef was first reported,
Marshall (1965) suggested "that [particulate] organic
matter, including aggregates, transported in suspension
from the reef to the mid-atoll areas, may constitute a sub-
stantial contribution to the trophic system within the
lagoon." Soon thereafter Johannes (1967) made additional
observations, noting a considerable flow of what he
referred to as organic aggregates as well as detrital frag-
ments streaming off the reef. His explanation, that the
aggregates were the remnants of coral mucus, flaking off
and flowing down-current from the rich reef environment,
probably also accounts for some of the material Marshall
had seen. Though Johannes provided additional quantita-
tive information, the concept that organic particles flowing
from the reefs might play an appreciable role in the lagoon
trophic system remained a matter of conjecture.

Marshall had yet another idea as to a potential source
of organic particles from off the reef, taking his lead from
the publications by Baylor and Sutcliffe (1963) and by
Riley (1963) in which they introduced the concept that dis-
solved organic matter in seawater might be aggregated
into particulate food. The Baylor and Sutcliffe paper had
demonstrated, in the laboratory, a mechanism that could
explain their formation; namely, that dissolved organic
matter in the presence of bubbles would aggregate and
thus form particles. They commented on the possible
importance of organic particles being produced by wave-
induced bubbles at sea.

Reflecting on the crashing waves at the seaward edge
of a typical coral reef, the streaming of bubbles in the path
of flow across the reef, and the very high organic produc-
tion of typical reef systems, Marshall envisioned a stream-
ing of organic aggregates forming on bubbles or other
nucleii. In observations on Hogsty Atoll in the Bahamas,
he found that amorphous organic particles were indeed
abundant in the lagoon but he could not establish a net

181

182

MARSHALL AND GERBER

gain as water crossed the reef (Marshall, 1968). Thus this
hypothesis concerning organic particle formation, attractive
though it may be, remains unproven.

Whether one denies or argues that such a bubble-
related aggregate formation mechanism is appreciable in
the flow over a reef, there is no denying the earlier and
continuing observations of detritus, mucus fragments, and
other organic particles in the water flow toward the
lagoon. In a very simple set of observations on corals held
in shipboard tanks, Marshall (1972) showed that stimula-
tion of corals by jets of seawater, intended to simulate the
effects of breaking waves, did increase the output of
organic particles. Qasim and Sankaranarayanan (1970)
demonstrated that particulate organic matter greatly
increased over the reefs of Kavaratti Atoll in the Laccadive
Islands. During Project SYMBIOS, headed by Robert
Johannes (Johannes et al., 1972) and with both Johannes
and Marshall present, the research team was looking for
striking examples of a rich mucus and aggregate flow, but
a visible "marine snow" consisting of these small
suspended fiarticles was not as evident as that observed
earlier by Johannes. Subsequently we have learned, from
observations by Gerber and by John T. Harrison 111 (per-
sonal communication) that the appearance of "snow" varies
from time-to-time and at differing sites behind the reefs.

Some quantitative information on such inputs, largely
from Enewetak data, was summarized by Marshall, Durbin,
Gerber, and Telek (1975). Also Johannes and Gerber
(1974) report on plankton-net detritus in the reef flow at
Enewetak, wherein they indicated a large percentage of
algal fragments.

Such work provided the background for Gerber to take
the necessary steps to explore the extent to which the
detritus and various amorphous particles in the flow from
the reef at Enewetak might be directly utilized by con-
sumers in the lagoon. At first he focused on gut contents,
analyzing a calanoid copepod, Undinuh uulgahs; a lar-
vacean, Oi/cop/eura longicaudata; and seven species of the
small pelagic fishes of the Enewetak Lagoon. For the two
zooplankton species, he noted that detritus and amorphous
p>articles predominated in the guts, while phytoplankton
cells were present in only trace quantities. Though the
small fish directly ingested reef detritus in the form of
suspended algal pigments and fecal pellets, the bulk of the
gut contents consisted of copepods and larvaceans. Cou-
pling these observations suggests a food chain in which a
base of detritus and aggregates is eaten by zooplankton
(Gerber and Marshall, 1974a and b), which then can be
eaten by the small fishes.

Later at Enewetak with the help of his wife, Mary,
Gerber quantified the particulate food requirements of
representative planktivores of the atoll system as he held
them in the laboratory in containers filled with water from
the lagoon. Ascertaining the particulate organic content of
the water before and after feeding and subtracting fecal
depKssits, the Gerbers obtained quantitative data on
material consumed and assimilated (Gerber and Gerber,
1979).

With this information and with the sampling of zoo-
plankton populations carried out by Gerber while doing
other work at Enewetak (Gerber, 1981), it was possible to
approximate total requirements as noted in Table 1 (con-
densed from Gerber and Marshall, 1982). Since there were
no assimilation assays for larvaceans to include in the
table, it was estimated, on the basis of comparisons of the
body content of carbon and nitrogen, that the requirements
of this group are about one-sixth that of the small
copepxxds (Gerber, unpublished data). Microorganism car-
bon requirements are based on summer respiratory rates
of concentrated suspended particles from windward reef
samples (Johannes ct al., 1972). Winter rates were
assumed to be about one-third the summer rates based on
relative abundances (Gerber and Marshall, 1982). Nitrogen
assimilation rates were estimated to be about one-eighth of
the carbon requirements, interpreting a ratio of 8:1 from
Vinigradov's (1953) chemical composition data.

TABLE 1

Assimilation Requirements of Particulate Organic

Carbon and Nitrogen by the Major Primary Consumer

Groups of the Pelagic Lagoon Environment*

Winter 1972

Winter 1974

Summer 1974

C N

Copepods 3.10 0.40 4.14 0.53 9.77 1.27
Pteropods 0 21 0.03 0.71 0.09 17.40 2.13
Larvaceans 0 28 0.04 0.37 0.05 0.97 0.11
Microor-
ganisms 1.10 0.14 1.10 0.14 3.30 0.41
Rate of as-
similation [4.69] [0.61] [6.32] (.81] [31.44] [3.92]

'Units are in mg m~^ d~' (condensed from Gerber and Marshedl,
1982).

Curious as to whether the reefs might supply an appre-
ciable portion of these requirements, we noted that the
windward reefs at Enewetak have a net trans-
port of seawater into the lagoon amounting to 13.2 X 10*
m'^ d^'* during the month of June (Atkinson et al.,
1981). This figure is multiplied by the quantity of particu-
late organic carbon (POC), 34 mg C m~^, flowing off the
reef [the average values for Enewetak samples collected on
glass fiber and silver filters (Marshall et al., 1975)]. t The

'Atkinson et al. (1981) give their data in terms of tidal cycles
of 12 h 25 min. For quantities per day we simply double their
data since none of the values are refined sufficiently to correct for
the 50 min difference involved.

t Other workers (Simmons, 1979, and Westrum and Meyers,
1978) have commented on the utilization of such particulate
organic carbon by the reef community and, in the
Westrum/Meyers paper, quite a point is made of high POC at
the reef crest with a rapid decline over the back reef. Our data
(Marshall et al., 1975) does not show this marked difference
between the reef and the back slope. For this discussion of input
to the lagoon we use the back slope veilues.

TROPHIC RELATIONSHIPS

183

product of these values divided by the volume of
Enewetak lagoon, 420.5 X 10^ m^ (Atkinson et al.,
1981), yields an effective reef input of particulate carbon
of 1.07 mg C m^'^ d ' in summer. The effective reef
input of particulate organic nitrogen (PON) is estimated to
be one-sixth of the carbon rate, based on the C:N ratio of
6.6:1 for the particulate organic matter exported from the
windward reef tract at Enewetak (Webb et al., 1975).

Because of the lack of entering data, values for the
flux of POC and PON across the windward reef in winter
are even more sp>eculative. Though winter and summer are
not differentiated as the data are presented by Atkinson et
al. (1981), our interpretation of the lumped values and
their range is that the currents across the reef, which as
they point out are driven largely by surf, would be three
times as great in winter when the trade winds prevail.
Since the increased surf and current must cause a greater
release of mucus from corals and of other detritus particles
of reef origin, we have suggested that the reef input of C
and N in winter is about three times greater than in sum-
mer, or >3 mg C m~'^. The fxjtcntial reef input is appreci-
ably increased if one speculates, quite reasonably, that at
least some of the dissolved organic matter flowing off the
reef (about three times that of the POC according to
Marshall et al., 1975) is aggregated into particulate form
available to consumers.

In winter such estimated inputs from the windward reef
alone would seem to meet the estimated C and N require-
ment of the lagoon primary consumers. In summer, when
reef inputs apparently are not as great yet consumer
demands may increase, this input seems to fall far short of
demand. Other sources that may be involved to meet
estimated consumer requirements would be:

1. POC entering from other reef areas (i.e., from
other than the windward sectors)

2. POC entering via the Deep Channel, the channel
toward the Southwest, and through other passes

3. POC from coral knolls— there are 2300 of these
with a total area of 9.8 X 10^ m^ (Emery et al., 1954)

4. Photosynthetic inputs

a. From plankton

b. From benthic macroflora

c. From benthic microflora

Since Atkinson et al. (1981) repjort no net inward flow
from across leeward reefs and through the passes, contri-
butions via (1) and (2) probably are not very large. Also,
since the depths of the crests of the coral knolls average
36 m below the surface and the flow is not great (2 to 4
cm s"' according to Atkinson et al., 1981), it seems
unlikely that there is a major input from that source.
Among the photosynthetic inputs, the role of benthic
macroflora must be minor since, as Gilmartin (1960) points
out, such vegetation is not abundant. Similarly one can
expect very little from benthic microflora in view of the
lagoon depths of about 50 m, though it is possible that
algae symbiotic in foraminifera, particularly in the shallows,
contribute significantly. At Takapota Atoll, Sournia (1976)

attributed high benthic productivity to such symbionts, and
Lee (1978) suggests that this can occur in low light; how-
ever, there are no observations directly relating such an
input at Enewetak.

This leaves photosynthesis by plankton as the likely
major additional POC source. Unfortunately, there is only
one set of determinations (Doty and Capuro, 1961), indi-
cating a production of 5.76 mg C m~^ d~' on a winter
day. This scant information suggests that phytoplankton
production may equal the combined inputs from the wind-
ward reefs and the other sources listed above. Further-
more, since Gerber and Marshall (1982) found that phyto-
plankton were more than twice as abundant in summer
than in winter, it seems likely that such production is a
major factor in meeting summer consumer requirements.
(No consideration is given to dissolved organic matter gen-
erated from productivity within the lagoon since this would
not constitute additional input but, rather, part of the pro-
duction, release, and reformation processes taking place
within that part of the system.)

To summarize, it appears that reef inputs constitute an
important part of the lagoon trophic regime, especially in
winter. Photosynthesis by the lagoon phytoplankton may
be at least equally important, with summer inputs probably
being the greatest. The average of consumer requirements
in summer is not as great as Table 1 suggests since peaks
in the abundance of pteropods and larvaceans (Gerber and
Marshall, 1982) and concentrations of jellyfish described by
John T. Harrison Hi (personal communication) are probably
of short duration.

Comments on Organic Fluxes of the
Atoll System as a Whole

The foregoing historical and narrative account of
organic inputs and utilization in the lagoon deals with only
one facet of the trophic relationships in the entire atoll
ecosystem. Considered in a more comprehensive way,
whether at Enewetak or elsewhere, the chief compart-
ments of a reef and the adjacent shallow water ecosystem
are the inputs from the

1. Surrounding oceanic waters

2. Outer reef slope

3. High reef

4. Knolls and patches in the lagoon

5. Overlying lagoon waters

6. Benthic environment of the lagoon *

These compartments, except for the oceanic waters, are
lumped by some authors under the inclusive heading:
"coral reef ecosystems."

It was mentioned previously that the inputs from sur-
rounding oceanic waters are low. It is admitted, however.

'For some reef settings there are also extensive mangroves
inshore of the lagoon or other coastal shallows, and these
represent an additional input to the system; however, mangrove
areas are not developed at Enewetak and generally are not exten-
sive on atolls.

184

MARSHALL AND GERBER

that for studies at Encwetak this has not been established
from direct measurements but is assumed from general
oceanographic considerations. Most of what we do know Is
inferred from sampling water as it comes in over the reef,
already referred to in Chapter 9 where the observations
of Odum and Odum (1955) and Johannes et al. (1972) are
discussed. These observations could be misleading since
they include an unknown net value for the uptake and the
release from the outer reef slope. They are, however, com-
patible with the generalization that substantial coral forma-
tions can develop in relatively impoverished waters (see
Lewis, 1977; Kinsey and Davies, 1979, for some of the
factors that bear on this). One point that can be made is
that, for an atoll system in an isolated setting surrounded
by ocean depths, these surrounding oceanic waters are the
only source of basic nutrients, except for the nitrogen fixa-
tion processes also referred to in Chapter 9

As mentioned earlier, the usual and plausible explana-
tion for reef growth under these conditions is that the reef
community, as a biological system, is uniquely adapted to
the uptake of low nutrient concentrations and attains high
gross productivity through recycling (e.g., Odum and
Odum, 1955; Pomeroy and Kuenzler, 1969; and Johannes
et al., 1972). It is often suggested that, in upwelling and
other enriched areas where nutrient levels are higher, the
success of competing ecosystems explains the general
absence or p>oor development of reefs. Except that multiple
responses and complications were involved, reef deteriora-
tion in Kaneohc Bay, Oahu, Hawaii, in the presence of
nutrient-rich sewage effluents (also the recovery there after
sewage diversion) seems to support this. [For discussions
of the Kaneohe Bay story see Banner, 1974, and Smith et
al., 1981.] Also, from fertilization experiments, Kinsey and
Davies (1979) suggest that nutrient concentrations can
suppress coral calcification. There docs seem to be a posi-
tive effect from nutrient replenishment, however, since at
Enewetak, as in most reef environments (Lewis, 1977), the
growth is most luxuriant on the windward side where the
greatest cross-reef flow occurs.

Unfortunately, very little has been done at Enewetak or
elsewhere to provide a direct insight into the organic pro-
ductivity of the outer reef slope. The nature of biological
processes on the slope are probably not substantially dif-
ferent in kind from those of the high reef. Rates are
undoubtedly reduced as light is attenuated with depth but,
as various workers have shown, this reduction does not
follow a direct linear relationship since there are some
accommodations to reduced light. Some limited observa-
tions on calcium carbonate production on the slope (Smith
and Harrison, 1977) suggest that, compared to the reef
flat, the slope input is minor. Sheppard (1982) provides a
comprehensive review of the little that is known about
slope environments throughout the world.

As noted in Chapter 9, the high reef has been inten-
sively studied. Generally speaking, gross productivity is
extremely high; net productivity is not. Thus it is the lim-
ited net productivity of this region, plus that of the less
productive outer slope and the input from relatively impov-

erished incoming oceanic water, that provides the suste-
nance described in discussing the trophic link from the reef
to the lagoon.

The productivity input of the coral knolls of the
lagoon, probably not great, and basic trophic relationships
of the lagoon waters have been explored in the section on
trophic links. As to the benthic environment, i.e., the
lagoon floor, an impressive feature is the abundance of
conspicuous consumer organisms. There are the calli-
anasids (ghost shrimps), with mounds so closely spaced
that there is often no level bottom between them, and sea
urchins (six identified species) in varying densities up to 80
m~^ (Colin and Harrison, 1981). Harrison (1983) notes
that more carbon is respired by the overall lagoon-bottom
community than is produced there; thus we must assume
that the bottom fauna must depend to some extent on fall-
out from the lagoon detrital and plankton complex.

This exercise, seeking to grasp the gross trophic rela-
tionships of the entire atoll system from fragmented infor-
mation, probably serves primarily to offer a sense of what
we do not know and need to learn. Even so, Enewetak
observations tend to conform to a generalization, discussed
by Kinsey (1979) largely from observations elsewhere,
which suggest that, considered cumulatively, the reef com-
ponents of such a system tend to be autotrophic, while the
remaining environments tend to be heterotrophic. Whether
this autotrophic/heterotrophic dualism balances out is not
known. Referring to reef systems in general. Smith (1983)
points out that we do not have a firm answer to this ques-
tion. Reflecting on the gross picture at Enewetak, we think
a balance does prevail, i.e., overall respiration equals or
offsets overall productivity. If this were not the case, one
would expect either an accumulation of organic matter on
the lagoon floor, in contrast to levels <1% (John T. Harri-
son 111, personal communication), or enriched oceanic
waters down-current from the atoll. Unfortunately the
waters flowing from the atoll have not been analyzed.
Perhaps the lack of noticeable pelagic fisheries concen-
trated down-current from atolls suggests that enrichment
there is not great.

Implications for Fishery Yields

Initially two considerations seem to imply a minimum
fisheries potential, in spite of the very high gross produc-
tivity of the extensive reefs and knolls. On the one hand,
many of the environments of the atoll system show little or
no net production. Also, as suggested above, the whole
atoll system seems to be in balance, with little or no
excess productivity. On the other hand, certain consider-
ations may offset this:

1. With systems so highly productive, even a small
percentage net release can be appreciable. Furthermore,
we have shown that such releases do occur and are being
utilized by consumer food chains within the overall system.

2. Noting that marine fisheries commonly occur in
regions that are in a climax or near-climax state, it is sug-
gested that capture fishery harvests may involve tapping

TROPHIC RELATIONSHIPS

185

the balanced (respiration equals production) systems or
altering their intracommunity trophic structures to some
degree, probably a bit of each. This raises a fundamental
fisheries question of general application as well as for
Enewetak: What level of harvests can be sustained through
tapping an otherwise climax system or by altering it?
Marshall has used the term "ecological sustainable yield
(ESY)"* in referring to the harvest potential in this sense.
(For a further development of this point see Marshall,
1979 and Marshall, 1985.)

Determining the yield potential as just discussed, i.e.,
through excess production, by tapping into the cycles of
balanced systems, or by altering the systems, is not possi-
ble under present methodologies. Even if rates for these
categories were well-known, the width of the confidence
limits and the variability expected for such basic steps in
the food web are of far greater magnitude than ultimate
yields. Consequently, attempted calculations of the latter
would be meaningless. Thus the only possibility for an
appraisal of how much can be taken, i.e., the ESY, is to
review actual harvest experiences. Summarizing data from
reefs and adjacent shallows elsewhere, Marshall (1985) has
suggested a generalized harvest potential of 4 to 5 metric
tons km plus miscellaneous gleanings from off the reef.
While this may represent a norm, some reports show
much higher yields. For example, for American Samoa,
Wass (1980) indicated 27 tons km"^ while Hill (1978)
indicated 12 tons km~^. It now appears that the potential
commonly may run well over 20 tons for some locales yet
be even less than 1 ton for others (Alcala and Luchavez,
1982; Alcala and Gomez, 1985).

Though the research done to date at Enewetak has
contributed very little to the yield question in any direct
sense, the atoll could be used for further meaningful stud-
ies by experimentally fishing replicate knolls in the lagoon
and critically observing the response to different fishing
pressures. As in any climax environment, a properly
managed harvest may serve as a culling process to the
benefit of the system. Such observations at Enewetak,
which has not been fished to any extent since early in the
1950s, could throw further light on this f>ossibility. Hiatt
and Strasburg (1960) offer a good foundation for such
research in a publication rich in information on feeding
habits and ecological relationships of Marshall Island reef
fishes. Johannes, who was so involved in promoting basic
ecological research at Enewetak, has become a leader in
compiling useful life history information, often stressing
insights gained from native fishermen (Johannes, 1978).

While the question of fisheries potential is a promising
area for study, we would not wish to raise undue expecta-
tions but would close by quoting Kinsey and Domm (1974)
who take a conservative view:

This is not to be confused with the maximum sustainable
yield (MSY) commonly used in fisheries and dealing with
recruitment/growth/mortality patterns for single, or small
numbers of interacting, species.

Coral reefs generally have been found to exhibit a high
turnover of carbon but a relatively small zero net gain.
Thus, while they have typically one of the highest known
naturally occurring levels of productivity. It is apparent that
they cannot tolerate any heavy cropping. Removal of
biomass not only involves the removal of carbon from the
system, but other accumulated and recycling elements.

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Chapter 1 1

Terrestrial Environments and Ecologx; of
Eneivetak Atoll

ERNST S. REESE

Department of Zoology, University of Hawaii
Honolulu. Hawaii 96822

INTRODUCTION

Enewetak Atoll is a necklace of 39 coral islands sur-
rounding a circular lagoon. The atoll is a coral limestone
cap about 1400m thick sitting on a pedestal of volcanic
basalt rising abruptly some 5000 to 6000 m from the sea
floor. Enewetak is about 50 to 60 million years old, having
its birth in the Eocene of the Tertiary Period. It is notewor-
thy that during this time, sea level was about 50 m below
the present level and that during the Wisconsin glaciation,
about 20,000 years ago, the sea level may have been as
much as 150 m below present sea level. What we do not
know is to what extent vertical tectonic movements cou-
pled with rates of coral growth match these sea water level
changes. There is evidence that at one time the atoll was a
raised coral limestone island. The physiography and geol-
ogy of Enewetak are discussed by Colin and Ristvet,
respectively, in Chapters 3 and 4 of this volume.

The dry land area of Enewetak Atoll is only about 2.5
mi^, about 6.5 km^, about 1600 acres, or about 647.5
hectares. The total land area is less than 4 m above sea
level. The 39 islands which comprise this dry land area are
distributed along the north, east, and south perimeter of
the atoll (Fig. 1). A single, small island, Biken, occurs iso-
lated on the west rim. The islands range in size and biotic
diversity from extremely small patches of coral rubble colo-
nized by sparse vegetation to the larger islands of Enjebi,
on the north rim, which is triangular in shape and mea-
sures about 1.2 km in size, and Enewetak, on the south-
east corner, which is elongate and measures about 1.3
km in size. The larger islands on the south rim of the
atoll — Ikuren, Mut, and Boken — support a forest of
mature coconut and Pisonia grandis trees and a
correspondingly richer biota; however, these three islands
together constitute only 0.45 km^ of land area. It is impor-
tant to note that the area of the lagoon, approximately
925 km^, is about 138 times larger than the total area of
dry land.

The terrestrial ecosystem of Enewetak Atoll is the
result of the dynamic interaction between the biota associ-
ated with the small dry land area and physical parameters
of the environment, especially the climate, soil, and
groundwater. The shrubs and trees, man, birds, rats, and
land crabs are among the more conspicuous elements of
the terrestrial biota, whereas climatic events, the soils, and
the availability of groundwater are the most important
physical components of the ecosystem.

CLIMATE AND WEATHER

The climate of Enewetak Atoll is determined by its
geographical location in the north central Pacific. The atoll
lies well within the northeast trade wind area. The meteo-
rological events affecting Enewetak and details of the cli-
mate are discussed by Merrill and Duce (Chapter 6, this
volume).

With respect to ecology, there are a number of highly
relevant aspects to the weather at Enewetak that deserve
mention. There are two seasons, the dry season of approx-
imately 4 months duration, December through March; and
the wet season of approximately 8 months duration
extending from April through November. All aspects of the
weather are tied into this seasonal pattern.

Unvarying high temperatures, high humidity, moderate
rainfall, steady easterly and northeasterly tradewinds, and
partial cloudiness are all modulated seasonally. Even tropi-
cal storms and typhoons, which are otherwise unpredict-
able from year to year, occur more frequently in the wet
season.

The average minimum temperature in the dry season is
23.6°C and 23.7°C in the wet season. The average maxi-
mum temperature in the dry season is 30.6°C and 31.8°C
in the wet season. The variance around these means is
very small (Table 1 in Merrill and Duce). The minimum
temperatures occur at night or during storms, whereas the
highest temperatures occur during the afternoon of the rel-
atively few cloudless days, particularly during the months
of August and September in the wet season.

Average temperatures, of course, are less important to
organisms than are extreme temperatures which may exert

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TERRESTRIAL ENVIRONMENTS AND ECOLOGY

189

direct physiological stress on thern. Extreme temperature
values for Enewetak are 21 °C and 34.4°C, and these
values are rare (Blumenstock and Rex, 1960).

These extreme temperatures, taken by themselves,
probably do not impose severe physiological stress on any
of the terrestrial organisms at Enewetak, providing water
and shade are available. Seeking shade and the intake of
water are well-known active behavioral processes of many
terrestrial animals. Moisture and soil conditions determine
the distribution and abundance of plants. Studies of the
physiological ecology of terrestrial organisms were not
undertaken at Enewetak.

There are, however, supporting observations. For
example, Coenobita land crabs and especially Birgus, the
coconut crab, discussed later, tend to be nocturnal or at
least crepuscular in their activity. On dark, wet, overcast
days, however, they are occasionally observed out forag-
ing. Conversely, during the dry season they are more
active on dark, humid nights of new moon or cloud cover,
little wind, and brief rain showers.

Humidity is affected by temperature and moisture. It is
maximal in the morning and decreases in the afternoon as
temperature increases. It is higher during the wet season.
Physiologically high humidity may have an ameliorating
effect on high temperature through evaporative cooling,
providing the organism can situate itself in a microhabitat
of shade and exposure to wind. Unfortunately supporting
data do not exist for terrestrial organisms at Enewetak.

Brisk, steady winds characterize the weather at
Enewetak perhaps as much as the high, unvarying temper-
ature and humidity. Trade winds blow from the east or
northeast about 95% of the year. During the latter part of
the wet season, August through October, wind direction is
more likely to shift from the southeast around to the
north. Wind speed is about 5.8 to 10.4 ms^^ (or 13 to 24
mih~ ). Winds are more brisk in the dry season and tend
to weaken in the wet season. Again selective exposure to
wind by an organism can ameliorate the effects of high
temperature and humidity. Seabirds while nesting on land
may position themselves to take advantage of wind direc-
tion (Lustick, 1984).

Partial cloudiness is the rule at Enewetak even during
the dry season, when, however, the degree of cloudiness is
more variable. The sky is seldom clear. Cloudiness
decreases solar radiation and affects the duration of time a
plant will be exposed to direct sunlight and, therefore, has
a moderating effect on terrestrial ecology.

The average annual rainfall at Enewetak is 1470 mm.
It is not distributed uniformly throughout the year. About
85% of it falls in the wet season starting in April and end-
ing in mid-November. October is the wettest month. The
remaining 15% falls in the dry season. There is consider-
able yearly variation. Needless to say, rain affects both
temperature and humidity, and cloud cover is greatest dur-
ing rainy periods. Situated in the extreme northwest, at
11°N latitude, Enewetak is one of the driest of the
Marshall Islands. Kwajalein Atoll (9°N latitude) averages

about 2400 mm, and the average annual rainfall at Jaiuit
Atoll (6°N, latitude) exceeds 4000 mm.

Perhaps the single most important aspect of rainfall in
the terrestrial ecosystem is the replenishment of ground-
water. The hydrography of Enewetak is discussed by
Ristvet (Chapter 4, this volume). The larger islands of the
atoll have a lens of fresh water of varying quality and
volume. Probably the distribution and abundance of vegeta-
tion and related biota on the larger islands are directly
related to the availability of groundwater. The correlation,
however, is not possible due to drastic alterations of the
vegetation resulting from activities during World War II
and subsequent events at Enewetak.

Although on average the weather at Enewetak is both
predictable and benign, at least in the general patterns
described previously, there are two aspects of the weather
which are remarkably variable and unpredictable. Both
have profound effects on the terrestrial ecosystem.

First, wind and rain squalls, wind shifts, periods of little
or no wind are of short duration and seem to occur almost
at random. They are not detected within the larger
weather pattern as measured periodically by conventional
weather recording instruments. Nevertheless, these events
are probably of great importance to terrestrial organisms in
modifying the effects of high temperature, desiccation,
humidity, and solar radiation. To my knowledge, measure-
ments to substantiate this statement have not been made
for the terrestrial biota of atolls.

Second, the occurrence and severity of tropical storms
and typhoons are highly unpredictable. Tropical storms of
greatest strength are called typhoons in the Western
Pacific. Of eight tropical storms and disturbances which
impacted Enewetak from 1959 to 1979, only one attained
typhoon strength (Table 2, Merrill and Duce, Chapter 6,
this volume). This was Alice, which struck Jan. 5 and 6,
1979. Three were tropical storms, whereas the remaining
four were classed on the basis of their severity as distur-
bances or depressions. What is even more remarkable is
that no weather disturbances occurred during the 7-year
period 1959 to 1966 and for a 3-year period 1973 to
1975. Of the eight tropical storms and disturbances which
did occur between 1967 and 1979, six occurred in the
wet season with three of these occurring in October, the
wettest month (no doubt these data are correlated), but
two occurred in January in the dry season.

The severest of these, Alice in January 1979, caused
the greatest amount of damage to the terrestrial environ-
ment of any storm that I observed over the 19 years,
1960 to 1979, that I visited Enewetak Atoll. Storm waves
coming from the east and northeast washed over the entire
north end of Enewetak Island (see frontispiece) carrying
away vegetation and flooding the laboratory buildings.
Wind speeds reached 145 km hr~^ Strand vegetation of
Ipomoea vines, Lepturus grass, and Tournefortia and
Scaevola shrubs was either washed away by high seas on
low-lying, small islets or denuded of leaves by the wind.
Massive rearrangement of sand and coral boulders

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REESE

occurred along the shore. The more densely vegetated
islands such as Ikuren Island on the southwest rim of the
atoll were greatly altered. Large Pisonia trees were
stripped of their leaves and uprooted; Tournefortia trees
were stripped of their leaves, many branches were broken,
but relatively few trees were uprooted. Cocos palms were
least impacted. The ability of palms to withstand typhoon-
force winds deserves study. Bunches of coconuts and
fronds were torn from the crowns, and, although a few
trunks had broken, 1 recall no coconut trees being
uprooted. By November 1979, the vegetation was making
a remarkable recovery with new growth appearing every-
where from the remains of broken plants. Short green
shoots covered the broken trunks and branches of
Tournefortia. Similar rapid recovery of vegetation occurred
following nuclear tests (Held, 1960).

Although surveys were not made, 1 suspect that the
land crabs and rats were not greatly affected by the storm
because they live in burrows and piles of debris, unless of
course, these were close to shore Held (1960) noted that
land hermit crabs of the genus Coenobtta survived the
blast and heat effects of nuclear explosions. Apparently
they were protected by the heavy shells of the marine gas-
tropod Turbo. Most likely rats, if present, survived too in
their burrows. The effect on insects and geckos which tend
to live on the vegetation must have been much greater.
Probably the birds were most severely affected by the
storm. Fairy and noddy terns nesting in trees and ground-
nesting scabirds would be greatly affected. A breeding col-
ony of sotty terns on a sand spit between Ikuren and Mut
Islands was completely washed away.

Overall, secondary ecological succession appears to be
the result of typhoons on the terrestrial vegetation of an
atoll. For example, in the early 1960s when I first visited
Enewetak and began my work on Ikuren Island, the vege-
tation was a dense shrub-like growth, 3- to 6-m high of
Scaeuola and Tournefortia under a canopy of tall coconut
palms. There were small meadows of Lepturus grass.
Much of the vegetation had been cleared during the testing
period in the 1950s, and what was evident was secondary
growth. The coconut palms, neatly planted in rows, dated
from the end of the 19th century when the Germans were
organizing copra production in the Marshall Islands. In the
1960s there was a small stand of Pisonia grandis trees in
the central part of Ikuren under the coconut tree canopy.
This stand covering an area of about 1000 m consisted of
trees about 10-m high with trunks not more than 20 to 30
cm in diameter. Gradually over the years the Pisonia forest
expanded until in the late 1970s it began to dominate the
aging coconut trees (Fig. 2). A Pisonia forest seems to be
the climax stage in the ecological succession of atoll vege-
tation (Lamberson's Stage V, Chapter 3, Volume II, this
publication). The large Pisonia trees, however, are
susceptible to storm damage. The wood is relatively soft,
the canopy large, and the root system poorly developed in
the shallow soil and rubble of the atoll. The trees are bro-
ken, uprooted, and blown over in tropical storms and
typhoons, resulting in a return to an earlier successional

stage. This is precisely what appears to have happened on
Ikuren Island during Typhoon Alice in January 1979
(Fig. 3).

For further details and observations on ecological suc-
cession of vegetation on Ikuren and other islands of
Enewetak Atoll, see Lamberson (Chapter 3, Volume II).
What is very clear is that the vegetation has suffered
repeated severe perturbation over the years, particularly
the northern islands, but with time it begins to recover.
Although diversity measures were not made, observations
indicated that diversity is higher in the early stages,
thereby supporting current disturbance theory (Loucks
1970; Miller, 1982, Sousa, 1980).

SOILS

The calcareous soils of Enewetak Atoll are similar to
those of other coral atolls (Fosberg, 1954; Fosberg and
Carroll, 1965; Hammond, 1969; Jamet, 1982; Mason,
1960; Niering, 1963; Seru and Morrison, 1985; Stone,
1951, 1953: Trudgill, 1979; Wiens, 1962). They are
relatively poor and immature consisting of limestone rub-
ble, sand, organic litter, and humus in various mixtures.
They have low moisture retention capacity.

If soils are defined in the broadest sense as the mate-
rial on the ground surface in which plants grow, then atoll
soils fall into five types:

1. Accumulations of coral rubble, mainly of stone size.

2. Unaltered coral sand and gravel.

3. Soils with a weakly developed A horizon with the
color only slightly darker than the unaltered sand below
but with no evidence of structural development. These
soils are exemplified by the Shioya series (Stone, 1951).

4. Soils with a more developed A horizon that is
deeper and darker in color than the Shioya type and with
some structural development. These soils are exemplified
by the Arno series (Stone, 1951).

5. Soils with an accumulation of raw humus on the
surface and with a relatively deep A horizon as in the
Jemo series (Fosberg, 1954). In the Jemo series, the accu-
mulation of humus is specifically related to the presence of
Pisonia grandis trees. There is an accumulation of phos-
phorus, often in the form of a cemented layer, believed by
Fosberg to be due to the reaction between guano from the
seabirds nesting in the Pisonia trees and the underlying
coral sand. There is some evidence that such a humus-rich
layer may develop under other suitable environmental con-
ditions as well (Catala, 1957; R. J. Morrison, personal
communication).

All of the soil types described previously would be clas-
sified under different names if conventional terms of soil
taxonomy were used (Soil Survey Staff, 1975). Atoll soils,
however, do not fall neatly into conventional classification,
and students of atoll soils find the five types described pre-
viously to be more useful.

Studies of the soils were not conducted under the
auspices of the Mid-Pacific Research Laboratory and, as a
result, detailed analyses of the soils of Enewetak Atoll are

TERRESTRIAL ENVIRONMENTS AND ECOLOGY

191

not available. Extensive radiological surveys were con-
ducted in the northern Marshall Islands, including
Enewetak, and provide information on the radionuclides in
the soil, vegetation, and animals (Robison et al., 1981,
1982).

The two most common and troublesome radionuclides
in the soils arc cesium-137 and strontium-90 because they
are picked up by plants, such as the coconut palm Cocos
nucifer, and are concentrated in the leaves and nuts which
may subsequently be consumed by man (Bastian and Jack-
son, 1975; Jackson and Carpenter, 1967). Radionuclides
have a cumulative effect in the diet: the more you eat, the
more you get. Surface material, soil, debris, and vegetation
containing these and other radionuclides were collected,
removed from the site of contamination, and entombed in
a slurry of concrete in two atomic craters at the north end
of Runit Island. Because of the transuranium nuclides,
chiefly plutonium, at this former test site and because of
the entombment of other radioactive materials on this
island, Runit is permanently off-limits to humans. It is
interesting to note that it took only a few years for
seabirds to recognize Runit as an ideal nesting site.
Seabirds returned to islands denuded by nuclear testing in
less than two years (Held, 1960). The people of Enewetak
prey heavily on birds and eggs, but they do not forage on
Runit. In 1985, B. Ristvet (personal communication) gave a
rough estimate of 10,000 birds nesting on Runit. He
reported that the smell of guano was perceptible about a
mile west of Runit in the lagoon.

Currently, at Bikini Atoll the United States government
is making an effort to eliminate the radionuclides from the
soil by means other than total removal of the contaminated
soil. The soil is relatively poor in potassium, and to com-
pensate, the plants pick up cesium-137. Adding
potassium-rich fertilizers reduces the uptake of cesium-137.
Although there is no clay in atoll soils which would serve
to trap cesium, adding a mineral silicate such as mica
tends to have the same effect. These findings offer hope
that a solution can be found short of soil removal for Bikini
Atoll. Fortunately, Bikini does not have the transuranium
nuclides found at Enewetak which necessitated the
extremely thorough cleanup of Enewetak Atoll.

From my observations at Enewetak, the A horizon
varies in thickness from a few centimeters to 40 to 50 cm
on the larger islands where it may be covered with a layer
of decomposing vegetation. The soils are usually well
drained and feel dry to the touch. Where they are poorly
drained, for example in depressions where the water table
is close to the surface, they have a wet sticky clay or
muck-like consistency.

On Ikuren Island the A horizon is about 40-cm thick on
the lagoon side. This part of the island is covered with a
dense growth of coconut and Pisonia trees. Small meadows
of grass, Lepturus repens, grow in open areas of the for-
est. Toward the ocean or south side of Ikuren, the soil
grades into coral rubble mixed with organic debris but
hardly any humus. This seems similar to the situation at
Bonriki Island, Tarawa Atoll, described by Seru and Morri-

son (1985). The coconut and Pisonia vegetation gives wau
to Scaevola and Tournefortia as the soil gets coarser. The
coral rubble becomes increasingly coarse until it ends
abruptly on a steep seaward berm of unconsolidated coral
rubble. The limestone rubble and sandy soil are typically
gray in color due to the blue-green algae, Brach]^thchia
quoyi, which may be important in nitrogen fixation (Nier-
ing, 1963; Wiens, 1962).

According to Trudgill (1979), there are three primary
sources of the soil of Aldabra Atoll:

1. Mechanically derived carbonate fragments

2. Chemically derived solution residues

3. Leaf litter

The composition of the vegetation and the phosphates
and nitrates derived from fecal material of birds, crabs, and
rats — all more abundant where there is more
vegetation — have considerable influence on the nature of
the atoll soils where the influence of organic materials is
especially significant. This is an important observation
because it means that the soil of an island such as Ikuren,
which has had good vegetation cover during the recent
past, should be richer than that of Enjebi and other islands
on the east and north rim of Enewetak, where the vegeta-
tion has undergone much disturbance during the past 40
years.

Soil is a precious terrestrial resource. In the atoll situa-
tion, the influence of organic material is all important. It
not only carries out the normal role of soil organic matter
in storing and recycling nutrients, but it is also the major
moisture storage component in the soils, since coral sand
and rocks have an extremely limited moisture storage
capacity. The fertility of atoll soils, therefore, is almost
entirely dependent on the content of organic matter (Seru
and Morrison, 1985). Every effort must, therefore, be
made to protect the organic-rich layers from erosion and
conserve them through cover of native vegetation.

TERRESTRIAL BIOTA (EXCLUDING MAN)

The terrestrial biota and ecology of Enewetak were not
studied as thoroughly as the marine ecosystem. From the
inception of the laboratory, the emphasis was placed on
marine organisms. The historical reasons for this are not
altogether clear (Helfrich and Ray, Chapter 1, this volume;
Hincs, 1962). In part it was because of the magnitude of
the marine environment when compared to the terrestrial
one and because there was an early concensus that a great
deal more was known about the terrestrial organisms than
about the multitude of unfamiliar marine organisms. In any
event, the opportunity to conduct research on a tropical
coral atoll apparently was much more appealing to marine
biologists than to other scientists. As a result far fewer
studies were made on the terrestrial biota and ecosystem.

We lack comprehensive, long-term studies of the plants
of Enewetak. What we do know has been summarized by
Lamberson (Chapter 3, Volume II). Ecological processes
have not been studied in the terrestrial ecosystem of
Enewetak. Other than species lists, we know very little

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REESE

Fig. 2 a, Ikuren Island looking west. The coconut and Pisonia forest is in the central part of the Island. Scaevola
and Toumefortia scrub vegetation is evident in the foreground; b. Coconut trees and sprouting nuts on Ikuren
Island. Clumps of Leptunis grass are seen growing on the coral rubble soil; c. Lagoon beach of Ikuren Island. At
night the land crab Coenobita perlatm forms courtship aggregations on this beach, and it is across beaches such as
this that the glaucothoe of Btrgus and Coenobita emigrate from the sea to the land. [Photographs by E. S. Reese.]

TERRESTRIAL ENVIRONMENTS AND ECOLOGY

193

Fig. 3 a. Typical beach vegetation consists of Lepturus repena grass, the flowering
morning glory vine Ipomoea pes-caprae. and the single whorl of leaves and small white
inflorescence of the shrub Toumefortia argentea; b. The interior of the forest on Ikuren
Island in the early 1960s when the Pisonia grandis trees were still 8m2dl in the fore-
ground. Note the sprouting coconuts. [Photographs by E. S. Reese.]

about the herptofauna and insects of the atoll. We know
essentially nothing of the role of the soil organisms
(Maguire, 1967). What little is known of the birds is sum-
marized by Berger (Chapter 13, Volume I, and Chapter
29, Volume U, this publication). We lack studies on the
behavioral and physiological ecology of the seabirds which
play such a profoundly important role in energy and nutri-
ent transfer between the marine and terrestrial ecosystems.
Only the land crabs and rodents were studied
thoroughly over a number of years. The research on the

rats and mice of Enwetak is described by Jackson et al. in
this volume (Chapter 12).

Land Crabs

Land crabs and birds are the most conspicuous animals
of the atoll. Of the land crabs, those belonging to the Fam-
ily Coenobitidae, the land hermit crabs, are the most con-
spicuous. Bright red to brownish red adult Coenobita per-
\atus axe found on most of the islands. Usually the adults
are found in Turbo shells, while the younger, smaller crabs

194

REESE

inhabit a greater variety of shells. Four other species of
Coenobita are present, but they are small and less colorful.
The legendary coconut crab, Birgus latro, the largest living
terrestrial invertebrate known, is nocturnal. It prefers dense
vegetation and is common only on the southwest islands of
the atoll from Ikuren to Biken. The brachyuran land crab,
Geograpsus crinipes, occurs but is secretive, preferring a
habitat of decaying vegetation in the forest. In contrast,
their relatives, especially the grapsid crab, Grapsus tenui-
crustatus, and the ocypodid ghost crab, Oc\jpode ceratop-
thalma, are active and conspicuous scurrying over the
intertidal beachrock or digging their burrows in the beach
respectively. These species are semiterrestrial only and are
not considered here.

At Enewetak, the behavioral ecology and life history of
the coconut crab, Birgus latro, were studied extensively by
Helfman (1973, 1977a, b), Reese (1965, 1968), and
Reese and Kinzie (1968); and the behavioral ecology of
Coer^obita spp. was studied by Held (1960), Page and Wil-
lason (1982, 1983) and Willason and Page (1983).
Osmoregulation, an important aspect of the physiological
ecology of land crabs, was studied by Gross (1964) and
aerial respiration by Cameron and Mecklenburg (1973).
Elsewhere land crabs were studied recently at Aladabra
Atoll, Indian Ocean, by Alexander (1979) and in the Mari-
ana Islands by Amesbury (1980). There is an excellent
account of the role of land crabs in the atoll ecosystem in
Wiens (1962). The discussion which follows is based on
these publications, literature citation therein, and my own
observations between 1960 and 1979.

Land crabs are tied to the sea for two reasons. First,
they release their fertilized eggs into the sea where they
go through typical crustacean larval stages in the plankton.
Second, their blood is isosmotic with seawater, and period-
ically they must have access to seawater to maintain this
condition. They are, nevertheless, surprisingly euryhaline
(Gross, 1964). Land crabs are scavengers on terrestrial
organisms, so most of their food presumably is less salty,
that is hypoosmotic, to their body fluids.

Birgus is extremely secretive and must be observed at
night with infrared viewing equipment if its behavior is to
be studied. Helfman (1977b) observed copulation in Birgus
on land, and at this time the spermatophore is transferred
to the female. It is not clear when fertilization actually
occurs. The eggs are carried by the female on her
picopods for about 3 weeks. In the case of Coenobita per-
latus, males tend to cluster around females on the beach at
night. They tumble and fight with one another trying to
gain access to the female. Presumably they are attracted
to her through chemosensory channels, but vision plays a
role too as rocks on the beach are also approached and
explored by males. Eventually one male wins access to the
female, both crabs partially emerge from their shells, ven-
tral sides together, and the spermatophore is transferred to
the female. She then proceeds into the water. It appears
likely that a ripe batch of eggs are hatched at this time,
the larvae are released, a new batch of eggs are extruded
to the pleopods, and fertilization occurs. Verification of

these events is necessary. Matthews (1956) was unable to
find adaptations for terrestrial fertilization in either Birgus
or Coenobita.

At Enewetak, Birgus latro females carry eggs from
about April through August. Initially the eggs are deep
purple red. As they develop on the pleopods of the female
they gradually turn lighter until just before hatching they
are light, translucent brown. The dark eyespots of the
embryos are visible at this time. With remarkable timing,
the female crab goes to the shore, walks out into the
water, flexes her abdomen repeatedly, and the thousands
of eggs hatch into free-swimming, first-stage zoeae larvae.

The larvae go through three more free-swimming
stages, four zoeal stages in all, before metamorphosing into
a postlarval stage called a glaucothoe. Reese and Kinzie
(1968) provide diagnostic features to distinguish the glau-
cothoe of Birgus from those of other Coenobita species.

The glaucothoe is a critically important life history
stage for these crabs. It is at this time that they carry out
a unique behavioral program. The glaucothoe, about 4-mm
long, settles to the bottom and begins to look for a small,
empty gastropod shell. It explores the shell using typically
hermit crab patterns of shell exploratory behavior (Reese,
1962, 1963). Then, with its newly acquired shell, the
Birgus or Coenobita glaucothoe crawls out onto the land
(Reese, 1968; Fig. 4). Shortly thereafter it undergoes a
second metamorphosis to a miniature crab. The abdomen
becomes asymmetrical in typical hermit crab fashion.
These tiny creatures are found in the high beach zone usu-
ally under rocks or debris. The wrack of the high-tide line
is a good place to look for them.

As they grow, they move inland. Coenobita crabs never
give up the behavioral characteristic of living in empty gas-
tropod shells, and, indeed, the availability of sufficiently
large shells may limit the population of large adult crabs.
At Enewetak, Turbo arg^/rostomus is the shell most used
by large coenobitid crabs. Many of the shells are broken
and worn and appear to have been in use for a long time.
The shell must be able to hold a small reservoir of water
apparently to keep the reduced gills and vascularized sur-
face of the gill cavity (sometimes called a pseudolung in
land crabs) moist. Close examination of empty shells found
in the jungle reveals a smooth, round hole in the ventral
whorl of the shell making it unsuitable for holding water.
The hole seems to be caused by solution from within
rather than wear from without.

The coconut crab Birgus lives in shells only when it is
small. Crabs reared in the laboratory abandoned shells
after about 2 to 3 years when their carapace measured 1
to 2 cm in length. During this time they were nocturnal
and secretive. Small crabs of this size are extremely diffi-
cult to find in the forest on islands such as Ikuren. What is
important to note is that Birgus stop living in gastropod
shells at a very small size when suitable shells are still
available to them. Reports of large coconut crabs living in
shells or even in coconuts are misidentifications. In most
cases that I am familiar with, the crab is Coenobita breui-
manus which, like Birgus, is often bluish in color.

TERRESTRIAL ENVIRONMENTS AND ECOLCX3Y

185

Fig. 4 a. The glaucothoe of the coconut crab Birgus latro emigrating from the sea to the land; b, The same on a U. S.
dime for scale; c, A large adult coconut crab Birgus latro sitting among coconuts. The carapace may attain a width of
about 15 cm, and the legs may extend to nearly 1 m; d. An adult Coenobita perlatus In a worn Turbo shell sitting in the
crouch of a Toumefortia tree. [Photographs by E. S. Reese.]

Both Birgus and Coenobita are opportunistic
scavengers. They eat animal and vegetable remains as well
as fruits and probably bird eggs. I have seen them feeding
on dead birds and fish on the beach and dead rats in the
forest, and they are readily attracted to almost any kind of
human food. Coenobita quickly walk upwind to a garbage
dump and may even walk into the warm coals of a camp-
fire to retrieve food. Birgus is more secretive and prefers
to take food to its burrow.

The crabs do climb trees for unknown reasons; how-
ever, Coenobita especially climb into Scaeuola and

Toumefortia shrubs in which noddy terns are nesting, and
model eggs were found with scratch marks on them.
Coconut crabs climb coconut trees, but they have never
been observed by Helfman or Reese to cut down coconuts.
They are unable to open green coconuts. They do, how-
ever, open brown coconuts on the ground. Usually the nut
is completely husked, and the reddish brown fiber is often
conspicuous at the entrance to a burrow (Fig. 5). The crab
apparently pierces the soft eye of the coconut, the one
through which the young plant will emerge, and then with
its powerful cutting claw cuts open the nut. To my knowl-

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REESE

Fig. 5 a, A burrow of the coconut crab, Birgus latro. Note the partially shredded
coconut and above it the fiber at the entrance to the burrow: b. Land crabs Coenobita
perlatus are active scavengers. They are abundant on Ikuren Island. [Photographs by
E. S. Reese.]

edge, the entire sequence of events has not been
observed. There are a number of questionable accounts in
the literature. It is important to note that the broken line
on old nuts found in the forest always passes through one
of the eyes. This is not true of the clean break made by a
man with a machette. Rats can gain access to nuts by
gnawing through the tough fiber and into the soft eye of
the nut. Rats do not shred the fiber from the nut, and,
therefore, nuts opened by rats are readily distinguishable
from those opened by crabs. Rats can gnaw into green

nuts in the crown of the tree. Coenobita are unable to
open coconuts and must rely on what is left by Birgus and
rats.

The population of coconut crabs on Ikuren Island was
studied periodically from 1960 to 1976 using tagging
recapture methods by Reese, Helfman, and their col-
leagues. These data will be part of a monograph on Birgus
latro which is in preparation. The population size on
Ikuren ranged from a low of 300 crabs estimated in April
of an exceedingly dry season to a high of about 1200 to

TERRPSTRIAL ENVIRONMENTS AND ECOLOGY

197

1400 crabs during the rainy wet season. Because of the
life history characteristics of Birgus emigration, immigra-
tion, natality, and morality are not considered to affect
these estimates Adult crabs cannot move between islands.
Small, young crabs are secretive, slow growing, and not
numerous in the data. Large adult crabs are estimated to
be 30 to 40 years of age. Therefore, the differences in
population estimates arc postulated to result from foraging
behavior. The best explanation of the data is that during
ideal conditions of moisture and lush vegetation, the crabs
forage every night, while under adverse, dry conditions,
they forage only every third to fourth night.

The ratio of females to males is nearly even. Males are
larger than females. Crabs are solitary. Small crabs defer
to large crabs when feeding.

Four species of the genus Coenobita occur at
Enewetak. Coenobita perlatus is the most abundant and
conspicuous species. The large red adults remain in the
forest during the day but go to the beach at night to for-
age, to replenish the water in their shells, and to repro-
duce. They prefer dark nights. A full moon tends to inhibit
their activity. Small C. perlatus occur at all times closer to
the beach; and their numbers are associated with the
amount of debris and cover present on the beach. They
may be tied closer to the beach by osmoregulatory
demands. Coenobita rugosus is common too, and both
large and small individuals tend to occur farther inland.
They are less common on the beach, and these were gen-
erally females engaged in releasing their larvae into the
sea. Coenobita breuimanus is not common. It tends to
occur deeper in the forest often closely associated with
Birgus and indeed has been confused v;ith Birgus. Gross
(1964) suggested the C breuimanus may be more depend-
ent on fresh water for shell reservoir replenishment than
the other two species. Coenobita cavipes is rare. The avail-
ability of suitable, empty gastropod shells appears to limit
the population of crabs on smaller islands such as Bokan-
dretok when compared with larger islands like Ikuren.

Coenobitid crabs are scavengers like Birgus, and Page
and Willason (1983) demonstrated that they play an impor-
tant role in reducing carrion and thereby potential fly
breeding sites. They also feed on fruits, flowers, roots, and
seedlings of a wide variety of plants.

Land crabs play an extremely important role in atoll
ecosystems. Alexander (1979) and Fosberg (Wiens, 1962)
observed that crabs carried seeds from the beach into their
burrows, thereby effectively planting them, providing they
were not eaten. They also noted that on atolls where
Coenobita scavenged and removed carrion, flies were not
abundant, whereas on atolls where Coenobita were scarce,
flies were common. Flies lay their eggs in rotting organic
material, especially carrion. In addition, the burrowing
behavior of land crabs tends to mix and aerate the rubble
and poor soil of the atoll.

On uninhabited islands adult Birgus latro reign as the
dominant terrestrial animals. They are vulnerable to rats
and insects at time of molting only, but this is done under-
ground affording some measure of protection. Man is the

principal predator on adult coconut crabs and probably
adult Coenobita as well in the atoll ecosystem. Where
human populations are high, crab populations, especially
populations of Birgus. considered a delicacy, are low.

Rats

Although the biology of Enewetak rodents is treated
elsewhere in this volume (Chapter 12, Jackson et al), it is
important to attempt to evaluate the impact of rodents on
the atoll ecosystem. In general, rats are considered de-
structive to island ecosystems (Smith, 1969; Wodzicki,
1969, 1972). They are a major problem in coconut and
sugar cane plantations. The least offensive is the Polyne-
sian rat, Rartus exulans, considered a commensal with
man; it probably accompanied, most likely as a stowaway,
the early Micronesians on their voyages of discovery. The
roof rat, Rartus rartus, and the Norway rat, Rattus norueg-
icus, are larger and do more damage. Fortunately, at the
time Jackson and his colleagues conducted their surveys
(1964 to 1978) only the roof rat was present at Enewetak.

Temme (1982) examined the stomach contents of 602
Polynesian rats collected in the northern Marshall Islands
during the wet season, October and November 1978,
including 243 from five islands of Enewetak. By estimated
volume, about 98 to 99% of the diet of R exulans is of
plant origin. The remaining 1 to 2% is animal matter, prin-
cipally insect remains. It should be noted, however, that
the cellulose of plant material and the chitin of insect parts
are more readily detected in stomach contents than, for
example, the remains of a bird's eggs. Temme noted that
those islands which were free of rats had the largest bird
populations.

At best, rats may contribute to the atoll ecosystem by
digging burrows, thereby helping to aerate the soil, and by
feeding on carrion, including human excrement, thereby
reducing the potential breeding sites for flies. At worst,
because they enter human habitations, they may be vec-
tors for disease, they compete with man for plant food,
and, in all probability, they attack the eggs and young of
nesting seabirds. The latter may be their most destructive
role because seabirds play such an important role in pro-
viding organic replenishment of atoll soils. Seabirds arc
perhaps the single most important group of organisms pro-
viding an energy bridge from the marine to the terrestrial
atoll ecosystem.

MAN AND THE ISLAND ECOSYSTEM

Carrying Capacity

Man is the dominant biotic component of the terrestrial
ecosystem. This is especially true for small, isolated,
self-sustaining ecosystems such as islands. Indeed, the
concept of carrying capacity of Pacific islands for human
papulations is the subject of considerable interest and con-
cern (Bayliss-Smith, 1975; Carroll, 1975; Kirch, 1980;
Kiste, 1974). The consensus is that island populations had
reached their full potential size before contact with Euro-
peans. In the Marshall Islands, ownership of land is of

198

REESE

great importance both economically and socially (Kiste,
1974). Carroll (1975) refers to "homeostasis in the precon-
tact populations" in his study of the population of Nukuoro
Atoll. It is probable that Pacific islanders were well aware
of the dangers of overpopulation, and homeostatic popula-
tion controls were actively practiced. Carefully controlled
infanticide was a primary mechanism (Bayliss-Smith,
1975).

Taro, Colocasia esculenta, is regarded as the essential
staple in estimating carrying capacity because it is the only
substantial source of starchy carbohydrate. Abundant pro-
tein from the sea seems to have been of less importance.
In almost all cases, European contact resulted in sharp
declines in island populations due largely to disease.

Little is known of the populations of the Marshall
Islands, much less individual islands, before European con-
tact (Hezel, 1983; Howe, 1984; Kiste, 1974), but it is
estimated that at the time of European contact in the mid
to late 1800s the population of all the Marshall Islands was
about 10,000 inhabitants. Today there are about 35,000
inhabitants.

With respect to Enewetak, it is interesting to ask what
the p)opulation size was in the past, especially before the
major disturbances of World War II. In the late 1800s
Hager (1889) reported that there were about 40 inhabi-
tants living on Enewetak, probably representing a severe
post-European contact decline in the atoll's population. By
1896, Irmer reported 60 natives on the atoll. According to
a British Naval Intelligence publication of 1945 (Naval
Intelligence Division, 1945), the population in 1935
amounted to 81 natives and 13 Japanese. This figure is at

variance with a figure of 121 inhabitants in 1930 given by
Emery et al. (1954). Kiste (1974) provides comparable fig-
ures for nearby Bikini Atoll. Although Japanese traders
lived on the atoll at this time and copra was being pro-
duced, the Japanese administration was based in Ponape,
and in all likelihood most of the sustenance of the native
population was based on the indigenous food resources of
the atoll. At the end of World War II, it is said that there
were about 130 Enewetak people living on Enjebi Island
where they had been moved by the Japanese. Other fig-
ures are 136 and 141 people in 1944, at which time they
were again living in their two traditional communities
located on Enewetak and Enjebi Islands. I am unable to
verify these figures with references. According to R. C.
Kiste (personal communication), there were 141 people in
the Enewetak community in 1947. Based on all these fig-
ures, it appears that the carrying capacity of Enewetak
Atoll is about 125 to 150 human beings.

The food web, energy-flow pattern for the Enewetak
ecosystem showing major food sources for man, especially
from the terrestrial environment, is shown in Fig. 6 and is
based on dietary information from Domnick and Seelye,
1967; Muri, 1954; Naidu et al., 1981; Niering, 1963;
Robinson et al., 1980; Wiens, 1962. The diet pattern de-
picted is that of a community (Naidu et al., 1981; Robin-
son et al., 1980) which is characterized as follows:

1. Maximum available local foods

2. Highly depressed local economy — living within
income provided by selling copra

3. Low population

4. Little or no ability to buy imported food

Pigs
Chickens

Sea Birds

Fish

Invertebrates

Turtles

MAN

Other Plants

Land Crabs
Pigs

Rats

Coconut

Breadfruit
Pandanus

Tacca
and Taro

Coconut Crabs
other Land Crabs

^ Organic
Debris

Soil
Organism

Insects —" Geckos

\
Chickens

Fig. 6 Food web, energy-flow pattern for the Enewetak ecosystem showing major food pathways to man. Marine organisms are
to the left and terrestrlid organisms on the right side of the diagram. It has not l)een possible to show all pathways. For example,
seablrds and their eggs are consumed by land crabs, rats, and man. Coconut crabs are eaten by man. Only plants are shown as
contributing to organic debris, but waste products from animals as well as their remains also contribute to organic debris.

TERRESTRIAL ENVIRONMENTS AND ECOLOGY

199

Needless to say, as the human population increases, the
availability of natural, subsistence foods will decrease, and
there will be an increased dependence on a cash economy
and imported foods. At this point, the atoll ecosystem,
with man as an integral part, has exceeded its carrying
capacity, and further degradation of the ecosystem will
occur unless the deficit is balanced with imported
materials.

In this regard, the observations of Domnick and Seelye
(1967) on Majuro Atoll in 1967 are extremely interesting.
Their tentative conclusion, based on an admittedly small
sample of nine families over a period of 30 days, is that
even the highest income families adhere to a subsistence
diet in preference to commercial foods. They believe this is
largely due to the cost of canned foods. When commercial
foods are inexpensive, they are used extensively. For
example, rice "is almost essential at every Marshallese
meal." Tea is preferred to coffee because it is less expen-
sive. Large amounts of sugar are consumed. Copra is the
single most important source of cash.

Clearly, the present human population of over a
thousand persons clustered on the three large southeastern
islands of Enewetak, Medren, and Japtan far exceeds any
historical population of the atoll and no doubt far exceeds
the natural carrying capacity of the atoll. In fact, the pres-
ent population is almost entirely dependent on subsidies of
food and material goods. In my judgment, it is doubtful
that the present population could ever be self-sustaining,
even with cash from copra, an expanded fishing program,
handicrafts, and perhaps even tourism.

Disturbance by Man

The terrestrial environment has undergone a series of
increasingly severe man-made disturbances. These are sum-
marized in Table 1. Probably significant environmental
change began with the planting of coconut plantations for
copra production under the supervision of the German
colonial government, 1885 to 1914. Although there was
no German administrator resident on the atoll, copra
freighters entered by the deep east channel and anchored
in the lee of Japtan Island. Soil from north Germany, car-
ried as ballast, was offloaded onto Japtan as copra was
loaded. The extent of this operation is unknown, but Jap-
tan Island is 1 to 2 m higher than the other islands and
supports lush vegetation. Presumably soil organisms were
introduced, but they have not been studied. Foreign visi-
tors were discouraged during this time.

Following the defeat of Germany in World War 1,
Japan took control of the Marshall Islands under a man-
date from the League of Nations, 1914 to 1944. Copra
production continued, and Japanese traders resided on the
atoll. In 1939 the Japanese began to construct military for-
tifications on Enewetak including an airstrip on Enjebi
Island. Several thousand Japanese military personnel lived
on the atoll. According to Hines (1962), the Japanese gar-
rison on Enewetak numbered 2686 armed troops and
about 1000 other personnel in January 1944.

Environmental perturbation must have increased dra-
matically during the war years culminating in the bombard-
ment and capture of Enewetak by American forces in
February 1944. Aerial bombardment coupled with naval
gunfire, land-based artillery, and the effects of small arms

TABLE 1

Chronology of Man-Made Disturbances of the Terrestrial Environment of Enewetak Atoll, Marshall Islands

1. About 2000 years ago human beings settled in Micronesia (Craib, 1983), presumably including Enewetak Atoll.

2. Discovery Era, 1526 to 1885 (Buck, 1953; Emery et al., 1954; Hines, 1962; Kiste, 1974; Sharp, 1960).

a. The first European to visit the Marshal! Islands was the Spanish explorer Alvaro dc Saavedra in 1529. He landed at an island,
the description of which fits Enewetak, on Oct 1, 1529, Other Spanish galleons sailed through the islands during the 16th cen-
tury. There are few details and no way to know whether or not Enewetak was visited

b. Sir Francis Drake aboard the Golden Hind may have visited the Marshall Islands in 1579. Then, for the next 200 years,
apparently no Europeans visited the Marshall Islands.

c. In the 18th century, a number of famous European explorers arrived In 1767 Samuel Wallis in HMS Dolphin rediscovered the
northern Marshall Islands from old Spanish charts. In 1788 he was followed by Captain John Marshall for whom the islands
were named Apparently Marshall visited only the southern islands Whether Enewetak was visited is unknown.

d. Enewetak Atoll was rediscovered on December 13, 1794, by Captain Thomas Butler commanding the British sloop Walpole. He
named the uncharted islands Browne's Range, and he apparently also referred to Enewetak as Walpole's Island In his journal.
He did not land. Captain John Fearn aboard the Hunter is said to have surveyed and charted the atoll in 1798. Enewetak was
often referred to as Browne or Brown Atoll even during World War II.

e. From a scientific standpoint, the most important expedition was commanded by Otto von Kotzebue aboard the Rurick in 1816
to 1817. He called them the Ratak Islands and believed he had discovered them (Kotzebue, 1830). Adelbert von Cha-
misso, an extremely competent naturalist, was a member of the expedition. He made the first observations of the geology and

(This table continued on next page.)

200

REESE

TABLE 1 (cont'd)

natural history, including the first chart of the Marshall Islands. The surgeon and zoologist Frederick Eschscholtz made zoological
collections Enewetak Atoll in its extreme northwesterly position seems to have been missed; however, he visited Bikini Atoll
which he named Eschscholtz Island Kotzebue again visited the Marshall Islands for further exploration in 1824
In 1841 Lt. Charles Wilkes, commanding the U. S. Exploring Expedition in the Peacock and the F/ying Fish, visited the north-
em Marshall Islands Charts were made of some of the atolls along with valuable observations on their natural history. For
example, while chartering Rongerik Atoll, Wilkes observed no coconut or pandanus trees and saw no humans There were other
visits during the second half of the 19th cpntury, particularly by whaling ships
The first missionaries arrived in the Marshall Islands in 1857 It is not clear when their influence was first felt at Enewetak.

11.

12.

German Protectorate. 1885 to 1914.

No Europeans lived on Enewetak Atoll during this period The German administration was on Ponape The Germans encouraged the
Enewetak people to grow coconuts for copra, which they sold to German traders This had the effect of shifting the Enewetak society
from a subsistence economy to a mixture of cash and subsistence
Japanese Mandate, 1914 to 1944

The Japanese seized Enewetak and all other German possessions in Micronesia in 1914 Subsequently, they continued to control the
islands under mandate from the League of Nations Although a Japanese trader and two assistants resided on Enewetak, the adminis-
tration continued to be from Ponape A number of Japanese scientific parties visited the Marshall Islands, but little happened until
1939 when the Japanese began to fortify Enewetak including building a landing field on Enjebi Island There were several thousand
Japanese military personnel living on Enewetak from 1941 to 1944.

United States Forces captured Enewetak in February 1944 Heavy aerial and naval bombardment preceded the battle which lasted
several days, from February 17 to 22 The Battle of Enewetak was the last assault against a defended atoll in World War II
At the end of World War II, the United States was given trusteeship of the Micronesian Islands, formerly under Japanese control, by
the United Nations

In December 1947, the Enewetak f)eople were transferred to Ujilang Atoll. At this time the population was about 141 people.
From 1948 to 1958, the United States undertook a series of 43 nuclear tests at Enewetak From 1958 to 1977, the atoll was used
for other quasi-military purposes. This was a time of major environmental disturbance to the atoll. Buildings, testing facilities, roads,
and airfields were constructed The human population fluctuated from several dozen to several thousand during this period, depend-
ing on the testing oper.-ition.

In 1954 the Enewetak Marine Biological Laboratory was established It was operated by the University of Hawaii from 1954 to 1983
with funds allocated by the Division of Biology and Medicine of the U S. Atomic Energy Commission. The name of the laboratory
subsequently was changed to the Mid Pacific Marine Laboratory and later, to further reflect the scope of its operation, to the Mid-
Pacific Research Laboratory For a history of the laboratory and its operation, see Chapter 1 of this volume.

In 1972 the U. S Atomic Energy Commission began radiological surveys of the atoll in preparation for the rehabilitation and reset-
tlement of the Enewetak people.

In 1977 the United States Government began the radiological cleanup and rehabilitation of the atoll. The work was completed in
1979.
In April 1980, Enewetak Atoll was officially returned to the Enewetak people (see Chapter 2 of this volume for further details).

fire and flamethrowers almost denuded some of the
islands — especially Eniebi, Enewetak, and Medren — of
their vegetation In the 1960s and 1970s a single tail coco-
nut palm on Enewetak Island, which had a bend halfway
up its trunk presumably due to damage sustained in 1944,
was said to be the only coconut tree to survive that turbu-
lent period. From 1944 to 1980, Enewetak was under
U. S. trusteeship granted by the United Nations.

In 1947, the Enewetak people were removed to Uji-
lang Atoll, and the United States government began 10
years of testing of nuclear explosives on the atoll, 1948 to
1958. Again there was a major impact on the relatively
fragile biota due to the construction of the test facilities
and the 43 nuclear tests. Finally, in 1977 the U. S. gov-
ernment undertook a major cleanup of the atoll in prepara-

tion for its return to the Enewetak people (Kiste, Chapter
2, this volume).

From the standpoint of terrestrial ecology the most sig-
nificant aspect of the clean up operation was the removal
of radioactive soil from many of the islands on the north-
east rim of the atoll. The contaminated soil was interred
with a slurry of concrete in two nuclear craters on Runit
Island.

Only the five islands on the south rim of the atoll, lying
west of the deep south channel, and Biken Island on the
west rim were relatively unscathed by these events. On
Ikuren, Mut, and Boken Islands the gradual replacement of
the coconut trees, Cocos nucifer, planted in rows under
German influence and now nearing senescense, by Pisonia
grandis trees is especially evident.

TERRESTRIAL ENVIRONMENTS AND ECOLOGY

201

Man-made disturbances of the Enewetak ecosystem,
particularly those resulting from World War II and the sub-
sequent nuclear testing program, probably were as devas-
tating as any that occurred on any other Pacific island in
history. What is remarkable is that with time the ecosys-
tem has demonstrated an astonishing resilience In a mat-
ter of 20 to 30 years the vegetation and its associated
biota were capable of recovering at least to the early
stages of ecological succession, and on the islands on the
southwest rim of the atoll a mature Pisonia forest was
becoming evident within that time frame.

THE FUTURE OF THE
ENEWETAK ECOSYSTEM

The key to the future of the terrestrial ecosystem of
Enewetak is in the hands of man. Environmental degrada-
tion brought on by overpopulation, rather than contamina-
tion by radionuclides, is now the principal threat to the
Enewetak ecosystem. The people of Enewetak were not
exposed to radiation. During the nuclear testing program
they lived on Ujilang Atoll, albeit under conditions of hard-
ship, and did not return to Enewetak until the cleanup of
radioactive material on the atoll was completed. Although
islands like Runit and a few of the severely disturbed
islands on the northwest rim of the atoll will remain
uninhabitable in the foreseeable future, the major islands of
the atoll, especially the traditionally inhabited islands of
Enjebi, Japtan, Medren, and Enewetak, should once again
become attractive places for human habitation. Enewetak
Atoll probably will never again be a self-sustaining island
ecosystem in the sense of carrying capacity for its human
population, but it can become a productive and contribut-
ing part of the Marshall Islands. It will take careful plan-
ning, strong community leadership, including birth control
and family planning, and continued support from the
United States government to achieve this goal.

ACKNOWLEDGMENTS

I am indebted to a number of people who have helped
me with this chapter. Albert H. Banner, Iraneus Eibl-
Eibesfeldt, Robert C. Kiste, John Morrison, and Goro
Uehara were kind enough to read portions of this chapter.
Lori Yamamura typed the manuscript. Susan Nakamura
prepared the art work.

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Chapter 1 2

Biolog}; of the Rodents of Enewetak Atoll

WILLIAM B. JACKSON,* STEPHEN H. VESSEY,*
and ROBERT K. BASTIANf

'Department of Biological Sciences, Bowling Green
State Uniuersitt;, Bowling Green. Ohio 43403;
fOffice of Water Program Operation, Environmental
Protection Agency/. Washington. DC 20460

INTRODUCTION

Rodents at Enewetak were casually observed or occa-
sionally specifically studied during the nuclear test program
(1948 to 1958). However, the rats frequently were
misidcntified; no unified analysis was attempted, despite
their being the only resident mammals on the test islets.
When Jackson was invited to join the University of
Washington's resurvey expedition in 1964, the foundation
for more than a decade of studies by Bowling Green State
University staff and students was established. Ten graduate
students participated in these efforts, and data from their
theses and dissertations are included in this discussion.
The inclusion of one of these students (Temme) in the
1978 Northern Marshall Island Radiological Survey permit-
ted collection of additional specimens and data from other
atolls.

Origin and Distribution

Three rodent species are present at Enewetak. The
Polynesian rat (Rattus exulans) came with the early
Micronesian inhabitants to the atoll. The house mouse
fMus musculus) may have arrived with the Japanese
administrators before World War II, but major infusions
probably came with American activities. The roof rat
(R. rattus) apparently arrived with American forces during
or after the war. The Norway rat (R. norvegicus), though
present elsewhere in the Marsha'ls, has not been observed
or trapped at Enewetak (or Bikini).

Probably the Polynesian rat occurred on most islets
used by the Enewetak people for coconut culture, but the
combined effects of clearing and construction and the deto-
nation of test devices decimated many islet populations of
this species. It remains on the less disturbed, more densely
vegetated islets.

The roof rat has flourished on some of the heavily
impacted northern islets as well as the main atoll bases
(Enewetak, Medren). The survival of this species in Enjebi,
within the impact zone of a nuclear detonation (Mike) as
well as numerous atomic tests, is hypothesized by Jackson
(1969). In an Atomic Energy Commission (AEC) experi-
ment it was introduced to Ananij Islet, where it has flour-
ished; other introduction attempts were made during the
test program but apparently were not successful (Fig. 1).

The Polynesian rat and roof rat exist allopatrically at
Enewetak. Such separation is not total on other Marshall
atolls, and one Bikini islet has sympatric populations. The
house mouse was found on only three islets (Enewetak,
Medren, Japtan) but was in combination with roof or
Polynesian rats.

Cats and dogs existed in varying numbers on islets
inhabited by test or administrative personnel (Japtan,
Medren, and Enewetak, during our studies). While these
animals occasionally caught rodents, they did not seem to
have any impact on the populations. Monitor lizards on
Japtan caught some rodents, but the impact of such preda-
tion was apparently insignificant. Coconut crabs (Birgus
latro) were scavengers rather than predators and were
observed eating opened coconuts alongside Polynesian rats
on Igurin. Reef herons (Egretta sacra) may have been
predators, but we did not observe such behavior.

House Mice

Mice were found in buildings (occupied or unoccupied)
on three islets (Enewetak, Medren, Japtan), and we also
caught them regularly in grassland and shrub habitats on
these same islets. Detailed studies of house mice were con-
ducted on Enewetak and Medren islets (Berry and Jackson
1979; Berry et al., 1981). These two populations were
genetically different, both in terms of external morphology
(pelage) and allozymic variation. Mean heterozygosities (per
locus) for these two islet populations were high, 11.4%
and 10.9%, respectively. Such variability has been
exceeded only in Hawaii (Berry et al., 1981). Selection for
certain loci with age, which occurs in nontropical environ-
ments, was not observed in these populations. Berry
(1979) suggested that such high genetical variability could

203

204

JACKSON, VESSEY, AND BASTIAN

Fig. 1 The Polynesian rat. Rattus exulans, and the roof rat, Rattus rattus, are distributed allopatrically at Enewetalt; a.
The Polynesian rat is found in more densely vegetated habitats and is particularly abundant on the larger islands where
coconut trees were planted for copra production. (Photo by William B. Jackson of Japtan Island in 1971); b. The roof rat
is found in more disturbed open habitats, particularly the northern islands of the atoll. (Photo by William B. Jackson of
Enjebi Island, looking to the southeast, taken in 1967). The remains of the former Japanese airstrip is visible in the fore-
ground, and the nuclear test building, now demolished, is seen in the middle distance.

BIOLOGY OF RODENTS

205

be an adaptation to reduce intraspecific competition under
conditions where a variety of foods is readily available.

The mice were smaller in body size than mainland
forms (9 g vs. 15+ g); this was hypothesized by Berry
and Jackson (1979) to be an adaptation to the constant
high temperatures in a situation where predators are gen-
erally absent. Both populations were considered to be
western (rather than Asiatic) in origin, supporting their
hypothesized introduction after World War II.

The high density Polynesian rat population on Japtan
islet interfered with specific trapping efforts there, and no
comparable studies were possible. We did occasionally
observe different pelage forms there, suggestive of animals
having escaped from laboratory colonies maintained during
the test program on this islet and interbreeding with the
local population.

Two breeding peaks (January through February and
July through August) were observed. Average litter size
was 4.0.

Habitat Selection

The Polynesian rat is a ground-dwelling rat, although it
has extensive arboreal highways, well odor-marked,
through the low vegetation. Frequently palm fronds were
used, which we confirmed with direct field observations
under red-light conditions and later in a simulated environ-
ment in our home laboratory using fronds brought back
from Enewetak. Nests were rarely found but were con-
sidered to be under or among surface debris (e.g., piles of
coconuts) or in shallow burrows. We never observed their
feeding in the crowns of coconut trees, even where islet
distribution of rats was allopatric. This species was trapped
on the trunks of coconut trees and was found feeding on
freshly damaged small nuts on Japtan.

This rat was found on the more densely vegetated
islets but was absent from some of the smaller and/or
more remote islets, especially on the western side of the
atoll. We conclude, on the basis of conversations with
Enewetak elders, that before the atomic tests, these rats
were present only on those islets that had been regularly
used in the past for coconut harvesting.

On islets such as Enjcbi and Runit as well as Medrcn
and Enewetak, which initially were infested by Polynesian
rats (based on early observations and records), clearing and
construction activities and test device detonation (in the
case of Enjebi and perhaps Runit) eliminated them. These
islets are now occupied by only the roof rat. Whether
interspecific competition was a factor in these local extinc-
tions of the Polynesian rat is not known. Accidental
transport of rats by man (initially by the Micronesians in
their canoes, later by AEC and military personnel in supply
craft) is considered the primary mode of spreading rats
around the atoll. Both tidal flow patterns and abundance of
predatory fish reduce the likelihood of direct water
transport.

The roof rat infested coconut crowns when these trees
were present. Although they readily climbed available

vegetation, they traveled easily on the ground surface.
They used surface debris or shallow burrows for nest sites;
they also dug around bunker foundations. This species
prospered on the more disturbed islets, those having less
cover, but was absent from more remote islets having
minimal human activity during the atomic test program.

The Polynesian rat and the roof rat readily invade
structures, and almost all storage and inhabited buildings
were infested. The facility at Lojwa during the cleanup
operations had a particularly difficult time with invading
Polynesian rats. The initial population density was high,
and vegetation removal forced the concentration of surviv-
ing rats in a perimeter strip. As soon as buildings were
erected, rats took up residence.

Popuiation Density and Home Range

Live-trapping, mark-and-release studies were under-
taken for roof rats on Runit and Polynesian rats on Japtan.
Roof rat population density in this open, grass-sedge-shrub
environment was about 20 animals 11,000 m^^ or about
one animal 550 m~^ (Jackson, 1967). This density may
have been exceeded in subsequent years on Japtan with
diminished human disturbance; frequently, we caught two
rats in the same snap trap.

In these environments, the home range (as measured
by standard diameter) for roof rats was 67 m for females
and 100 m for males. For Polynesian rats, it was 50 m
(both sexes).

Food Habits

Rats, even though at the apex of the terrestrial food
pyramid, are largely vegetarians in this environment. In a
study of food habits. Fall et al. (1971) found arthropxsd
(insects, centipedes) remains somewhat more frequently in
roof rat than Polynesian rat stomachs (33% vs. 10%).
Additional studies at Enewetak and on the Northern
Marshall Islands Radiological Survey reaffirmed these pat-
terns; however, the volume of animal matter was small
(<2%) (Temme, 1982). Seasonal fruits and seeds, as well
as vegetative plant structures, were recognized in the
stomach contents.

Roof rats occasionally were seen at night foraging on
the beach and even out onto the exposed reef flat. Fish
(trapped in tidal pools) may well have been caught or
scavenged, though we did not observe this.

In a simulated predation situation with roof rats con-
fined to beach enclosures, we did, with starlight scopes
and under direct moonlight, observe rats enter ghost crab
{Oci>pode sp.) burrows, pull the crab out, dismember, and
eventually eat the crab. Often the eye stalks were the ini-
tial target of the rat.

When we captured rats in the vicinity of ground-nesting
tern colonies, we occasionally found bird remains in the
stomachs. We were unable to determine if this resulted
from predation or scavenging. In one instance, we found'
opened eggs in a f>ortion of a sooty tern colony and

206

JACKSON, VESSEY, AND BASTIAN

suspected Polynesian rat depredations. However, this was
an isolated, not a repeated, observation.

Reproductive Patterns

Necropsy data, assembled over 14 years and available
from all months except May and December, provide a
composite picture of breeding patterns. For Polynesian
rats, a bimodal increase in prevalence of pregnancy is sig-
nificantly correlated (r^ = 0.82) with spring and fall rainy
seasons. The fall period is longer, and the prevalence of
pregnancy reaches nearly 0 25 in October (Temme, 1981).
We hypothesize that the increased food supply (and
perhaps cover) associated with increased precipitation is
the basis for greater reproductive activity (Fig. 2).

Despite the general data correlations, some individual
islet variations were observed. If coconut trees were
present, pregnant rats were more likely to be found. How-
ever, sufficient data were not collected to delineate the
extent of these intra-atoll variations.

On the basis of embryo counts, average litter size for
Polynesian rats was determined to be 3.3 (Table 1). About

a third (36%) of the pregnant females were primiparous.
These reproductive data are similar to those obtained else-
where in the tropics for this species.

Most males (84%) had scrotal testes, and few young
males were caught early in the breeding seasons. Acces-
sory organs (e.g., seminal vesicles) regressed in size during
the nonbreeding January period and recrudesced in March.
However, the variations in seminal vesicle length and
prevalence of pregnancy were only weakly correlated (r =
0.42). Some effects of population density and stress are
suspected (Temme, 1981).

Data from roof rat populations followed very similar
patterns, showing the same summer and fall reproductive
peak (Table 1); however, litter sizes were larger, averaging
4.2 young.

Behavior and Population Regulation

An atoll provides an unusual opportunity to study
natural rat populations because of the number of similar
islets in isolated proximity. Population densities vary from
islet to islet but are typically higher than those on the

Enewetak Ato

mean sum: 1^70 mm

Fig. 2 Relationship between monthly mean rainfctll and previdence of
pregnancy of Polynesian rats (means 1964 to 1978). Sample sizes (tops
of bars) are based on mature fem<iles (perforate vaginal orifice)
(Temme, 1979).

BIOLOGY OF RODENT S

207

TABLE 1

Summary of Female Rat Reproductive Data from
Northern Marshall Islands

Winter

Spring

Summer

Fall

(Dec-Feb.)

(Mar.-May)

(Jun.-Aug.)

(Sept.-Nov.)

Totals

Polynesian rats'

No. with vaginal

orifices perforated

293.0

4140

437.0

318.0

1462.0

Percent pregnant

7.7

156

176

10.9

Embryos/female

3.3

3.2

3.4

3.3

Roof rats

No. witfi vaginal

orifices perforated

1490

405.0

323.0

180.0

1057.0

Percent pregnant

4.0

6.2

14.5

21.7

105

Embryos/female

4.4

4.5

3.7

4.2

•From Temme (1981).

mainland; dispersal is nearly absent. In this context, Krebs,
Keller, and Tamarin (1969) have shown that blocking
dispersal leads to unusually high densities in some rodent
species.

Adrenal gland weight has been widely used to assess
the role of agonistic behavior in inducing a physiological
stress response. Laboratory and field studies have demon-
strated a positive correlation between adrenal gland weight
and both population density and loss of fights (reviewed by
Christian, 1978). Other work has demonstrated inhibitory
effects of crowding and aggression on reproductive func-
tion and disease-defense mechanisms. In this Enewetak
study (1977 to 1978), the relationships between popula-
tion density, adrenal size, wounding, and parasite load
were examined.

Trapping

Conventional Victor kill traps, baited with fresh
coconut, were set at approximately 7-m intervals before
sunset, emptied and rebaited several times during the
night, and pulled at midnight or the next morning. Because
of the variability in trapping effort, results are expressed in
rats caught per trap hour rather than per trap night.

Necropsy

Routine body measurements and reproductive data of
scrotal males and sexually mature, nonpregnant females
were noted; adrenals were preserved in 10% buffered for-
malin and later cleaned and weighed wet. The ratio of the
combined adrenal weights to the head and body length
was used in all subsequent calculations. Small intestines
were removed and also fixed in buffered formalin; later
they were opened and tapeworms counted. After clipping
hair from the posterior one-third of the back, remaining
hair was removed with a depilatory; fresh wounds were
counted.

Population Density and Adrenal Weight

On five Polynesian rat islets, with densities ranging
from 0.06 to 0.58 rats caught per trap hour, no consistent
relationship between population density and adrenal weight
was apparent for either sex.

For five roof rat islets, with densities ranging from
0.006 to 0.062 rats per trap hour, correlation analysis
revealed a significant, positive relationship between density
and adrenal weight in both sexes for this species (Fig. 3).
Enewetak Islet had a low population density, because of a
control program, and the lowest adrenal weights. Medren,
with large numbers of abandoned buildings, supported the
densest population and the heaviest adrenal weights. Other
islets were intermediate. Population densities on Enjebi
increased from 0.029 in 1977 to 0.045 in 1978, probably
because the island was chained between samples, and the
cut vegetation was placed in piles as part of a cleanup pro-
gram, thus concentrating the survivors. Adrenal weight/
body length ratios correspondingly increased from 0.28
(males) and 0.27 (females) to 0.40 (males) and 0.44
(females) (Table 2).

Population Density and Wounding

The mean number of back wounds per rat (species
combined) increased as population density increased (Fig.
4). Males generally had more wounds than females, partic-
ularly on the densely populated Polynesian rat islets of
Bijire and Aomon. On low density, mainly roof rat islets,
sex differences were not great.

Wounding and Adrenal Weight

Polynesian rats with fewer than two wounds had signifi-
cantly lower adrenal weights than did those with two or
more wounds (t-test, P < .001, both sexes; Table 3). For
the less frequently wounded roof rats, both males and
females with no wounds had significantly lighter adrenals
than did those with wounds (t-test, P < .025).

208

JACKSON, VESSEY, AND BASTIAN

Z .55

-I .50

Q
O
GQ
s -40

■

^ .35

OC .30

< .25

.20

f
m

f f
m
m

12 3 4

RATS/ TRAP HOUR

Fig. 3 Rats per trap hour (X 100) and adrenal weight/body length ratios for roof rats plotted
separately by islet (1977 and 1978); f = females, m = mjiles.

TABLE 2

Summary of Population Density Estimates

and Adrenal Gland Weights for Roof

Rats at Enewetak Atoll

. Adrenal wt./

Population'

body length

Islet

Males

Females

1977

Enewetak

0.58

0.250

0.278

Enjebi

2.92

0.275

0.268

Ananij

3.55

0.388

0.405

Runit

3.67

0.300

0.330

Medren

6.23
1978

0.387

0.438

Enewetak

1.01

0.269

0.276

Ananij

3.75

0.349

0.298

Enjebi

4.49

0.401

0.471

Medren

5.52

0.532

•Rats/trap hour X 100.

Parasites and Adrenal Weight

Two parasites were examined in the rats. The most
common was a stomach worm of the genus Protospiura,
which was present :n both species. The incidence or bur-
den of this parasite was not related to adrenal weight. The
other parasite was eui intestinal tapeworm, Hi/menolepis
diminuta, which was common only on Aomon. Rats of

both sexes which had tapeworms had heavier adrenals
than did those without tapeworms (t-test, P < .05;
Table 4).

Discussion

The results generally support the idea that as popula-
tion density increases so does the incidence of fighting, an
observation made many times in simulated free-living
laboratory colonies and in field observations under red
lights and with starlight scope. As a result of fighting, cer-
tain individuals — the losers of these fights — undergo a
stress response leading to increased production of glu-
cocorticoid hormones and hypertrophy of the cortex of the
adrenal gland. Although these hormones prepare the body
for fight or flight, they also are associated with a decrease
in reproductive hormones and the suppression of the
body's defense mechanisms. We have not examined the
relationship between adrenal hormones and reproduction;
but rats that were under stress were more likely to have
tapeworms, suggesting the predicted inhibition of defense
mechanisms against disease. The failure to find an effect
with the stomach nematode may have been because this
parasite, while residing in the stomach, does not attach to
the host or otherwise stimulate an inflammatory or
immune response.

The failure of the Polynesian rat to show a consistent
adrenal response with islet population density is difficult to
explain. However, this species typically lives at much
higher densities and seems more tolerant of crowding than
the roof rat. Although fighting is common among Polyne-
sian rats, as evidenced by simulated free-living laboratory

BIOLOGY OF RODENTS

209

TABLE 3

Summary of Enewetak Atoll (All Islets. 1978)

Wounding and Adrenal Weight Data for

Roof and Polynesian Rats

2 4 6 8 10 12 14 16
RATS/TRAP HOUR

Fig. 4 Rats per trap iiour (X 100) and mean wounds per rat
(roof and Polynesian rats combined) for eacii islet (1978); f =
female, m = males.

colonies and direct field observations and the incidence of
wounding, possibly these fights are not as severe or stress-
ful as with the roof rats.

Although it is likely that food sets the upper limit to
density on these islets, with the absence of dispersal and
predation as regulatory mechanisms, it also seems likely
that physiological changes associated with crowding act to
adjust birth and death rates to keep numbers below the
point where starvation occurs.

Stomach Parasite Loads

The stomach nematode (Protospiura muricola) was not
uniform in distribution. Although present in all Polynesian

No. of
wounds

No. of
animals Sex

Adrenal wt./
tx>dy length

P(t-test)

Rattus exulans

39
44

F
M

0.142
0.103

2 or more

55
45

F
M

0.192
0.144

<0.001
<0.001

Rattus rattus

47
28

F
M

0.341
0.323

1 or more

20
26

F
M

0.414
0.411

<0.025
<0.005

TABLE 4

Summary of Tapeworm Infestations in Polynesian
Rat Relative to Adrenal Weight, Aomen Islet

Sex

No. of
rats

Tape-
worms

Adrenal wt./
body length

P(t-te8t)

Male
Female

9
9
4
9

No
Yes
No
Yes

0.111
0.142
0.100
0.138

<0.05
<0.05

*Aomen Islet, Enewetak Atoll.

rat populations studied by P. C. Rabalais at Enewetak
(average prevalence = 0.32; Table 5), it was not found at
Bikini Atoll and Mejit Island In the Northern Marshalls sur-
vey (Temme, 1979). The parasite load was small in both
studies, about two parasites f>er rat.

In contrast, this parasite was not found in all roof rat
populations at Enewetak Atoll; Ananij and Enewetak rats
lacked the worm (Table 5). However, the average
prevalence on the other islets was 0.56. The average
parasite load was 10 times that in the Polynesian rat.

A second stomach nematode (Gong\ilonema neoplasti-
cum) was found by Temme (1979). Because it is imbedded
in the mucosal lining. It Is not often recorded. It was found
In nearly half the rats In all populations studied (Table 5).
However, the parasite load averaged 1.7 worms per rat.
About half the rats were Infested with both nematodes.

In the case of both host species, the larger animals
tended to have more parasites. Some roof rats had in
excess of 75 Protospiura. Even so, these parasites did not
appear to be a serious stress factor.

210

JACKSON. VESSEY, AND BASTIAN

TABLE 5

Summary of Stomach Nematode Distributions
Relative to Host Species, Islet, and Atoll

Species and
parasite

Islet

No.
rats

No.
infected

%
infected

Enewetak Atoll

R. rattus

Enewetak 14

(Protospiura

Runit 92

20.7

muricola)

Enjebi 56

83.9

Ananij 45

Medren 58

84.5

Totals 266

115

43.2

Totals (infected islets) 206

115

55.8

R. exulans

Japtan 105

28.6

(Protospiura

Lojwa 20

55.0

muricola)

Bijire 34

50.0

Aomon 44

27.2

lituren 59

20.3

R exulans
(Protospiura
muricola)

(Gongiilonema
r\eoplasticum)

Totals 262 84 32.1

Northern Marshall Atolls

602 141 23.4

602 208 46.5

•Temme, 1979.

Responses to Testing Program

During the Atomic Energy Commission's testing pro-
gram, detailed or systematic studies on rodents were not
conducted, although several individuals made brief observa-
tions and even transplanted rats from islet to islet. In some
cases, it was possible to reconstruct the experiments and
even correctly identify the rodent species involved. How-
ever, it was not until the 1964 Resurvey Program,
directed by the University of Washington, that continuing
studies of the rodent populations were initiated.

At that time, roof rats inhabited the larger, highly dis-
turbed islets; Polynesian rats inhabited the less disturbed
islets. Our radionuclide investigations focused on the roof
rat populations on Enjebi and Runit, although we studied
populations, including Polynesian rats, on other islets.

On Runit we examined roof rats at varying distances
from Cactus crater at the north end of the islet (Bastian
and Jackson, 1975). The rats concentrated the radionu-
clides they obtained from plants in their diet. At the
crater, levels of ^^^Cs ranged up to about 2000 pCi g^^
(dry weight) in plant tissues; in rat tissues, to 5000 pCi
g~' (Table 6). At the south end of the islet (where no test-
ing was done) radioisotope levels were close to back-

TABLE 6

Average "'Cs Levels (pCi g~' Dry Weight) of

Soil Samples and Plant and Animal Tissues

Collected on Runit Islet (1967)*

Distance from Cactus Crater, m

200

1030

1710

2460

Surface soil

344

10.8

3.7

0.5

Scaeuola fruit

437.5

56 1

7.5

20.4

1.7

Tournefortia leaves

2174.0

76.8

49.0

30.4

2.0

Roof rat liver

2261.0

276.0

38.8

11.0

3.5

Roof rat kidney

5134.0

722.0

95.6

38.0

4.7

'Numbers of samples vary. Data from Bastian and Jackson,
1975.

ground. We felt, on the basis of these data, that rats
would make excellent radiation monitors.

Further efforts to demonstrate the potential monitoring
role of rats led to the use of thermoluminescent dosimeters
(TLD), which were implanted under the nap>c skin of rats
that were released and then recaptured 5 months later
(1977 to 1978). Of the 185 implants made, 39 were
recovered, even though cleanup operations were under
way, which greatly interfered with the initial trapping and
recapture operations. Rats of both sfiecies on six islets
were involved. Exposures of 3 to 7 mR d~^ were recorded
from rats on Runit and about 1 mR d~' on Enjebi are con-
trasted to zero readings in Igurin and Ananij rats. These
radiation levels detected by the rats' dosimeters appear to
correlate closely with the measurements obtained during
the cleanup operations (Table 7).

The aborted PACE program (1972) had a principal
impact on Bijire with the removal and windrowing of plant
debris. The Polynesian rats used this increased cover, and
the sooty terns shifted their breeding colony to some of
the cleared habitat. From examination of stomachs, we
know that rats were preying or scavenging on the terns.
Numbers of Polynesian rats on the three-islet chain
(Aomon-Bijire-Lojwa) continued to be high during and fol-
lowing this period.

When the final cleanup program was organized, Lojwa
was set up as a base facility (1977), and most of the land
area was scraped clean. Polynesian rats were pushed into
the fringe of remaining vegetation. As soon as living quar-
ters were constructed, rat infestations became chronic
problems.

Garbage was disposed of in a pit across the bridge to
Bijire. Although periodic covering occurred, rats abounded.
We observed rats moving from distances of several hun-
dred meters to the dump, but no detailed marking and
movement studies were possible.

The cleanup on Enjebi resulted in chaining and wind-
rowing much of the vegetation and subsequent removal of
contaminated soil and debris from many areas. The roof
rats, perhaps reduced in total numbers, were concentrated

BIOLOGY OF RODENTS

211

TABLE 7

Summary of Ambient and Rat-Implanted Dosimeter Exposure
Results, Enewetak Atoll, November 1977 to April 1978*

Ambient dosimeters,
exposure (mR day ')

Implanted dosimeters,
exposure (mR day^')

Islet

No.

Mean

Range

Species

No.

Mean

Range

Enjebi

063

0 26 to 1 29

R rattus

089

0.41 to 1.78

Runit

4 12

2 94 to 6 51

456

3.29 to 6.86

Ananij

Bijiri

0.00 to 0 20

R. exu/ans

0.08

0.00 to 0.18

Aomon

0.27

0.12 to 0.43

0.13

0.00 to 0.20

Igurin

'Dosimeters supplied and analyses provided by I Aoki, Radiological and Environ-
mental Sciences Laboratory, Idaho Operations Office, DOE. Calculated results in mil-
lirem (mR) were derived for total exposure [seriod.

tToo low to calculate.

in these vegetation piles. Burning, however, was incom-
plete; resprouting of shrubs occurred rather quickly. The
rats increased in numbers and began to spread.

The bulldozing of transects for radiation measurements
on other islets probably had limited impact on the rats.
The brush piles provided increased harborage; certainly,
the carrying capacity of these islet habitats for rats was
not decreased.

In our earlier studies, we had found no abnormalities in
rats that we could associate with elevated radiation levels.
The rats were not larger, nor did we observe a greater
prevalence of tumors or resorbed embryos. Color phases
were not associated with radiation patterns. The melanistic
roof rats on Runit are better explained through genetic
drift in an isolated population. In recent studies, however,
Temme (1981, 1986; Temme and Jackson, 1978) found a
positive relationship between background radiation levels
and frequency of palatal ridge deviations in the roof of the
mouth (Figs. 5 and 6). For example, the incidence of
abnormal palatal ridges was 0.44 in Polynesian rats from
Lujor Islet, which had the highest test contamination of
islets still harboring this species. On Japtan, with no direct
testing contamination, the incidence was 0.06. Other popu-
lations were intermediate, but similar relationships were
not evident in roof rat comparisons.

However, when mean measures of divergence for
palate ridge variation are compared, values between
Enewetak and Bikini Atolls and between Enewetak and
other atolls are much larger than statistics comparing the
north end of Enewetak Atoll (greatest radiation exposure)
with the southerly islets (least exposure) (0.75 to 2.0 vs.
0.20 to 0.35) (Figs. 7 and 8). Thus these palate variant
patterns also may be influenced by geographic isolation —
the greater the geographic separation, the larger the diver-
gence statistic (Temme, 1981). As far as could be deter-
mined, these slightly altered palatal ridge structures cause
the rat no difficulty; we do not know if a mutation is
involved.

Plasma transferrins were examined by Malecha and
Tamarin (1969) from roof rats collected on Runit, Enjebi,
and Medren. Five alleles were found, compared to only

12 3 4 5mm

I — ■ — ■ — ■ — ■ — I

papilla palatina (^fc^

diastemal or

antemolor
ridges

\.v

intermolar ridges ii^fA/''' "SJ

— till >'*''V,. Lin

gulor pad

5 mm
' ' (^ 29.77,

^11.6 7.

Fig. 5 a. Nomenclature of palatal ridges in typical Polynesian
rat; b. Incidence of diastemal ridge deformation in rats on
Lowja (Enewetak Atoll), which had intermediate levels of
background radiation, was 0.30. On Japtan (outside the con-
taminated area) the incidence was 0.02. A new aberration,
involving intermolar ridges and found in 1256 of the speci-
mens, was unique to Japtan (Temme and Jackson, 1978).

212

JACKSON, VESSEY. AND BASTIAN

Boken 6184

Lujor /.329

Aomon 1981
/2257^

Enewetak 2.6

Fig. 6 The sections in the circles represent the percentage of abnormal antemolar palatal ridge pat-
tern in the Polynesian rat populations. The four islets with the highest percentages are centered
between islets with highest contamination data. These values given for several of the larger islets are
average fallout contamination levels in R/hr. corrected to H + 1 hr past detonation of the atomic
testing devices (Temme, 1981).

three in Hawaii. They suggested that the greater variation
in the Enewetak rats might be due to the nuclear detona-
tions.

In 1980 blood samples were taken from roof rats on
Medren (N=16), Enjebi (N=17), Runit (N = 35) for
analysis of protein variation. Of 29 loci checked by electro-
phoresis, 13 were polymorphic.

Heterozygosity (the percent of all loci per individual
that were heterozygous) was lowest on Medren (16.4),
intermediate on Enjebi (19.7), and highest on Runit (21.1).
The percentage of loci that were polymorphic also
increased in the same order, from 38% to 41% to 45%.
These findings raise the possibility that increased radiation
also may have increased selection for heterozygotes or
may have increased mutation rates on Enjebi and Runit.

Nei's (1972) genetic distance among the three islands
varies from 0.95 to 0.96. Comparable data for isolated
house mouse populations showed higher values, from 0.98
to 0.99 (Nei, 1972), suggesting greater isolation among
the rats at Enewetak.

SUMMARY

Roof rats and Polynesian rats, introduced to the atoll
by 20' century commerce and the Micronesians, respec-
tively, were present allopatrically on the larger islets. Of
necessity, they were largely vegetarians. Reproductive
cycles were keyed to rainfall patterns. High density popula-
tions had high stress indices, including high parasite loads.

The rats, at the top of the terrestrial food pyramid,
constituted a bioenvironmental monitor that was rarely uti-
lized during the several test programs. Bioconcentration of
radioisotopes, especially Cs and Co, occurred; rats
implanted with dosimeters were determined to function as
environmental radiation monitors. We hypothesized that
roof rats on Enjebi survived the nearby nuclear detonation.

Analysis of plasma transferrins indicated greater hetero-
zygosity in the northern atoll rat populations. The
incidence of oral palatal ridge deformations also was posi-
tively correlated with environmental radiation levels, but
other gross indications of radiation effect were not found.

BIOLOGY OF RODENTS

213

Enewetak

Fig. 7 "Mean measure of divergence" (X 100) for intermolar palatal ridge
variations in Polynesian rats between Islets at Enewetak AtoU (Temme,
1981).

Fig. 8 "Mean measure of divergence" (X 100) for Intermolar palatal ridge variations in Polynesian rats between Islets of
several atolis In the northern Marshall Islands (Temme, 1981).

214

JACKSON, VESSEY, AND BASTIAN

ACKNOWLEDGMENTS

Initial encouragement and involvement were provided
by Lauren Donaldson and other personnel of the Labora-
tory of Radiation Biology, University of Washington. The
laboratory analyzed many of our soil, plant, and animal
samples for radionuclides. Subsequent support was pro-
vided by direct AEC contract [AT (11-1)1485], MPML
(later Mid-Pacific Research Laboratory), various ERDA con-
tractors for additional soil and animal tissue gamma counts
as well as other atoll support services, and the Bowling
Green State University (BGSU) Faculty Research Commit-
tee. The most recent field studies were possible because of
the inclusion of Manfred Temme in the DOE Radiological
Survey of the Northern Marshall Islands (1978).

Many individuals have participated in these studies.
F. C. Rabalais (BGSU) contributed parasitological studies.
R. J. Berry (University College, London) with assistance
from the Royal Society of London conducted the house
mouse studies. W. Z. Lidicker and R. D. Sage (University
of California, Berkeley) carried out the mouse tissue
analyses. The dosimeters implanted in the rats were
prepared and interpreted by I. Aoki, DOE Radiological and
Environmental Sciences Laboratory, Idaho Falls. Roger Ray
and others of the DOE Las Vegas office were especially
helpful in obtaining data and information. William Robison,
Lawrence Livermore National Laboratory, also provided
information. Additional BGSU graduate students participat-
ing in the field studies included Michael Carpenter, Tom
Denbow, Michael Fall, Gail Haigh, Dale Kaukeinen, Robert
Lane, Willard McCartney, and David Rintamaa.

REFERENCES

Bastian, R. K., and W. B Jackson, 1975, "'Cs and '^Co in a
Terrestrial Community at Enewetak Atoll, Radioecology and
Energy Resources, Special Publication, The Ecological Society
of America, Fourth National Symposium on Radioecology,
Oregon State University, pp. 314-320.

Berry, R J , 1979, Genetical Factors in Animal Population
Dynamics, Population Dynamics. R. M. Anderson, L. R. Tay-
lor, and R. D. Turner (Eds), Blackwell, Oxford, pp. 53-80.

and W B. Jackson, 1979, House Mice on Enewetak Atoll,

J Mammalogv, 60: 222-225.

R. D. Sage, W Z Lidicker, and W. B. Jackson, 1981,

Genetical Variation in Three Pacific House Mouse (Mus mus-
culus) Populations, J Zoology. London, 193: 391-404.

Christian, J J., 1978, Neurobehavioral-Endocrine Regulation of
Small Mammal Populations, Populations of Small Mannmals
Under Natural Conditions, D P. Snyder (Ed), University of
Pittsburgh Press, Pittsburgh, pp 143158.

Fall, M W , A B Medina, and W B , Jackson, 1971, Feeding
Patterns of Rattus rattus and Rattus exulans on Eniwetok
Atoll, Marshall Islands, J Mammalogv. 52: 69-76.

Jackson, W B., 1967, Productivity In High and Low Islands with
Special Emphasis to Rodent Populations, Micronesica, 3: 5-13.

1969, Survival of Rats at Eniwetak Atoll, Pac. Sci., 23:

265-275

Krebs, C J., B L. Keller, and R. H. Tamarin, 1969, Microtus
Population Biology: Demographic Changes in Fluctuating
Populations of M. ochrogaster and M pennsvlvanicus in
Southern Indiana, Ecolog\i, 50: 587-607,

Malecha, S R , and R H. Tarmarin, 1969, Plasma Transferrins
In Three Species of Rattus on Pacific Islands, Amer. Nat.,
103: 664-669

Nei, M., 1972, Genetic Distance Between Populations, Amer.
Nat.. 106: 283-292.

Temme, M., 1979, Po/ynesian Rat (Rattus exulans) Populations in
the Northern Marshall Islands, Ph.D. dissertation. Bowling
Green State University, Bowling Green, Ohio.

, 1981, Reproductive Parameters of the Polynesian Rat

(Rattus exulans) In the Northern Marshall Islands, Z. ange-
wandte Zoologie. 68: 315-338.

, 1982, Feeding Pattern of the Polynesian Rat (Rattus exulans)

in the Northern Marshall Islands, Z angewandte Zoologie, 69:
463480

, 1987, Somatic Mutation In the Polynesian Rat (Rattus

exulans) at Enewetak Nuclear Test Site, in Current Mammal-
ogv. H. H. Genoways (Ed), 1: 483-493.

, and W. B , Jackson, 1978, Palatal Ridges as an Epigenetic

Marker In Rattus rattus and Rattus exulans Populations,
Z. Siiugetierkunde, 43: 193 203.

Avifauna of Enewetak Atoll

Chapter 1 3

ANDREW J. BERGER

Professor Emeritus. Department of Zoology
Uniuersiti/ of Hawaii
Honolulu. Hawaii 96822

INTRODUCTION

Birds constitute an important element of the Enewetak
fauna. They form a significant portion of the biomass of
larger land animals, and they also arc important foragers
of the surrounding ocean, particularly of the shallow reef
areas. The isolation of Enewetak— 200 km from the
nearest other land— dictates that the birds that have
arrived there have had to be strong fliers; this is especially
true for migrant spwcies.

At least 41 species of birds have been recorded at
Enewetak Atoll. Amerson (1969, and included in the EIS
of 1974) listed 32 species; Johnson and Kienholz (1975)
added three; Temme (1979) and Hailman (1979) added six
more. A checklist is included as Table 1. These birds are
described and illustrated by King (1967). Earlier papers
dealing with the avifauna of Enewetak were those of Gleize
and Genclly (1945), Woodbury (1962), Pearson and Knud-
sen (1967), and Carpenter et al. (1968).

No endemic species and no passerine species inhabit
the low islets of Enewetak. The islets presumably are too
low, too small, and too remote from major land masses to
have been colonized by birds that could have evolved into
endemic forms (Berger, 1979). Many of the native seabirds
are species that have a vast range in the Pacific region,
and many of them spend only the breeding season on land.
Of interest is the migration of the long-tailed cuckoo
(Eudi/namis taitensis) from New Zealand to the winter
range on Enewetak and other islands from the Bismarck
Archipelago eastward to the Marquesas Islands.

The bird fauna of Enewetak, however, is not well-
known, primarily because few observers have been able to
spend extensive periods of time on the various islets. Any
significant effort over a period of time would certainly add
other species to the list. For example, David Anderson
(unpublished), a former Peace Corps volunteer residing on
Ujelang Atoll, recorded 36 species of birds from that atoll
between June 1975 and February 1977; a number of
these species have not yet been recorded at Enewetak.

At least 12 species are known to breed on the atoll,
and at least four others are thought to nest there. Many
other birds probably include Enewetak within their range.
Owen (1977), King (1967), and Baker (1951) list a number
of species known from the Marshall Islands and other adja-
cent areas that have not yet been recorded at Enewetak.
The Marshallese names for the birds of Enewetak have
been listed by Goo and Banner (1963).

The importance of predators other than man on birds
at Enewetak is not well-known. Amerson (1969) suggested
that both the coconut crab (Birgus latro) and Coenobita
rugosa eat eggs and young birds. Reese (personal
communication) observed a coconut crab catch a white
tern by the wing and drag it into the crab's burrow under
coconut debris at the base of a coconut tree. The tern
apparently had been frightened from its perch by Reese's
night survey team. Certainly, land crabs are scavengers
and feed on bird carcasses when they are available.

On Igurin Island, however, Helfman (1973) did not see
coconut crabs feeding on either birds' eggs or young birds.
He noted that coconut husk and meat, plus a variety of
other items, were eaten. Igurin has large numbers of birds,
and if they were common prey of coconut crabs, this
almost certainly would have been observed. Amerson
(1969) and Berger (1981) report that both Rattus rattus
and R. exulans eat bird eggs and young. Medina and Jack-
son (MS) found no evidence for this, but Temme (1982)
reported that rats may have preyed on the eggs of Sterna
fuscata on Aomon. The monitor lizard, Varanus indicus, is
known to prey on birds and their eggs (Amerson, 1969),
but because it is virtually certain that the population previ-
ously present on Japtan no longer exists, this potential
predator is now gone. The return of the Marshallese popu-
lation to Enewetak again makes man the major predator
on birds at Enewetak.

Until the recent return of the Marshallese population to
Enewetak, the major effect of man on the birds had been
the alteration of the habitat by the fighting during World
War II and the subsequent testing activifles. The bird
population of Enewetak certainly suffered during the battle
in 1944 and thereafter when the atoll served as a staging
area for campaigns farther to the west. The coconut palms
and other vegetation were destroyed, and construction of
the airfield and support facilities drastically reduced the

215

216

BERGER

TABLE 1
Checklist of the Birds of Enewetak Atoll, Marshall Islands

Order PROCELLARIIFORMES

Family SCOLOPACIDAE (continued)

Family PROCELLARIIDAE

Heteroscelus breuipes

Polynesian tattler

'Puffinus pacificus

Wedge-tailed shearwater

Heteroscelus incanus

Wandering tattler

Puffinus griseus

Sooty shearwater

Arenaria interpres

Ruddy tumstone

Puffinus tenuirostris

Slender-billed shearwater

CalHnago hardwickii

Latham snif)e

Pterodroma hypoleuca nigripennis

Black-winged petrel

Calidhs alba

Sanderling

Order PELICANIFORMES

Calidhs acunriinata

Sharp-tailed sandpiper

Family PHAETHONTIDAE

Calidris ruficollis

Rufous-necked sandpiper

'Phaethon rubricauda

Red-tailed tropic bird

Tryngites subruficollis

Buff-breasted sandpiper

'Pbaethon lepturus

White-tailed tropic bird

Phihwachus pugnax

Ruff

Family SULIDAE

Family CHARADRIIDAE

'Sula sula

Red-footed booby

Charadrius dubius curonicus

Ring-necked plover

'Sula leucogaster

Brown booby

Charadrius mongolus

Mongolian plover

Family FREGATIDAE

Pluvialis dominica fulua

Pacific golden plover

tPregata minor

Great frigate bird

Pluvialis squatarola

Black-bellied plover

Order CICONIIFORMES

Family LARIDAE

Family ARDEIDAE

Sterna paradisaea

Arctic tern

'Egretta sacra

Reef heron

'Sterna sumatrana

Black-naped tern

Order ANSERIFORMES

•f Sterna lunata

Gray-backed tern

Family ANATIDAE

'Sterna fuscata

Sooty tern

Anas acuta

Pintail

' Thalasseus bergii

Crested tern

Anas querqueduh

Garganey teal

■f Procelsterna cerulea

Blue-gray noddy

Order GALLIFORMES

' Anous stolidus

Brown noddy

Family PHASIANIDAE

'Anous tenuirostris

Black noddy

■\Gatlus gallus

Jungle fowl or domestic

' Gvgis alba

White tern

chicken

Order CUCULIFORMES

Order CHARADRIIFORMES

Family CUCULIDAE

Family SCOLOPACIDAE

Eudynamis taitensis

Long-tailed cuckoo

Limosa lapponica baueri

Bar-tailed godwit

Order STRIGIFORMES

Numenius phaeopus

Whimbrel

Family STRIGIDAE

Numenius tahitensis

Bristle-thighed curlew

Asio flammeus

Short-eared owl

Tringa glareola

Wood sandpiper

'Confirmed breeding bird.

fBelieved to breed; confirmation needed.

ground cover. Many of the smaller islets remained basically
untouched during that period, but with the advent of the
testing program at Enewetak, major portions of the atoll
were swept instantly by destruction.

The effects of the nuclear testing on bird populations
has not been well documented, but certainly in those
islands swept by blast and heat, the decimation of the bird
pHjpulations must have been Inevitable. Hines (1962)
reports, for example, that after the first thermonuclear
explosion at Enewetak (test Mike) In 1952, on Rigili
(Leroy), some 14 miles from the blast site, "many of the
terns there were sick, some grounded and reluctant to fly
and some with singed feathers, particularly the noddy terns
and the sooty terns, whose feathers are dark in color." At
Bogallua (Alice) only 3 miles from ground zero, which pre-
viously had been "laden by stands of coconut palms and
thickly populated by birds" (Hines, 1962), no animal life
could be found. One week after the Mike test "transient
birds" were observed on Engebi in a scene of utter desola-
tion. Possible genetic effects on the birds apparently were

not studied. On Janet Island on July 23, 1971, Berger
found a white-tailed tropic bird chick with a deformed bill
in which the lower mandible protruded far to the right of
the normal upper mandible. This chick certainly died after
parental feeding stopped. In a large colony of sooty terns
on the same island, Berger found six immature terns with
such badly deformed left wings that the birds could not fly.
Nevertheless, environmental alteration has not been
totally negative with resp>ect to all bird populations on
Enewetak. The removal of vegetation from many of the
islands as a result of the cleanup program has op>ened up
new nesting areas for ground nesting birds, as documented
by Temme (1979).

FEEDING HABITS

No intensive studies of the feeding habits of the
scabirds of Enewetak Atoll have been published. Studies
have been made, however, on many of the species in
other parts of their range. For example, Ashmole and

AVIFAUNA

217

Ashmole (1967) wrote that squid (family Ommastrephidae)
and flying fish (Exocoetidae) were of primary importance
to a colony of red-footed boobies that nest on the island of
Oahu in the Hawaiian Islands. The same authors studied
the feeding habits of seabirds on Christmas Island (Pacific
Ocean) and concluded that flying fish and squid "are of
outstanding importance in the diets of nearly all species of
birds typical of the tropical Pacific." These findings were
confirmed by Shreibcr and Hensley (1976). Similarly, in his
study of the sooty tern in the Hawaiian Islands, Brown
(1973) found that the birds ate about half squid and half
fish by weight. Four families of fishes were found in the
diet, but the Carangidae (genus Decapterus) were the most
important. It seems likely that the seabirds of Enewetak
have similar feeding habits, even though some species tend
to feed in the lagoon and others range far at sea in their
search for food.

Drinking Saltwater

Fresh water is unavailable on most of the islands where
seabirds nest, and the birds are adapted to drinking salt
water. Not only do the birds not need fresh water, but
Frings and Frings (1959) discovered that captive black-
footed and Laysan albatrosses died unless they were fed
adequate amounts of salt. All seabirds that have been
studied possess special salt glands, located in the orbit,
that secrete a fluid that has a higher salt concentration
than that in seawater, thus leaving a net gain of water for
the birds' physiological needs. The hypertonic solution
that drains to the bill tip is discarded by head-shaking
movements.

Guano Production

The excrement of seabirds, known as guano, is rich in
phosphates and ammonium comp>ounds (largely ammonium
urate or uric acid). The input of nutrients of such guano to
island soils can be great in nesting areas. At Enewetak
there are recognizable deposits of bird guano in the Pisonia
stands on the southwestern islands, from Igurin to Rigili.
Richardson (MS) reported that at Rongelap Atoll there was
"a relationship between the greatest concentration of
breeding birds, the most extensive stands of large trees,
and the best-developed soils." On Kabelle Island he
estimated that the 1400 to 1800 birds of three tern
species contributed over 40,000 kg yr~' of waste to this
island of 2400 square meters.

Murphy (1936) said that a single Peruvian booby (Sula
uahegatus) produces as much as 150 g d~' of guano, and
that, if 30 g of this were deposited on an island, a
thousand birds would produce more than 10 metric tons a
year. He added that Peruvian booby guano is more than
33 times as effective as barnyard manure. However,
whether the guano accumulates in large quantities or is
leached dep>ends on climatic conditions, that is, the amount
and pattern of rainfall. Nevertheless, the guano input has a
profound effect both on the plant life of the islands and on
the lagoon and ocean surrounding them. For example,

Hutchinson (1950) describes these effects as follows: "If a
bird colony is situated on such a coast or island, part of
the upwelling nutrients will finally be deposited on the
island. Wherever any of this material is washed back by
rain or wave action into the sea, there will be locally and
momentarily a much greater concentration of nutrients
where the guano solution returns to the ocean than at any
other place. The result of a very large bird colony on a
section of coastline or on an island, whenever climatic con-
ditions and the form of the substrate of the colony permit
guano to be returned to the ocean, will be to steepen the
nutrient gradient. Nutrient elements that, without the
birds, would tend to remain in the bodies of the fish in the
fseripheral part of the trophophoric field of the colony and
on the death of the fish would presumably be distributed
widely throughout the general circulation of the ocean are
concentrated by the birds on and around the island. The
result will be increased littoral productivity and probably
increased littoral fish production. If the latter occurs, the
birds will not have to move so far out to sea for their
food, and a steady-state condition will be set up. . . . It is,
in fact, conceivable that large bird colonies, far from reduc-
ing the commercial catch of fish by competition, may actu-
ally increase the catch by a process of biogeochemical con-
centration." Also related to this situation are the excreta
voided in flight by the many thousands of seabirds as they
fly over the lagoons and the ocean on their way to and
from the feeding grounds.

BREEDING HABITS

Many of the seabirds or oceanic birds are called pelagic
species because they spend most of the nonbreeding sea-
son on the open ocean, returning to islands only for the
nesting season. Some sf>ecies do not reach sexual maturity
for several years (e.g., until about 5 years of age for the
frigate bird and black-footed albatross), and they may
remain at sea during this pericxl. Certain species (e.g.,
brown booby, sooty tern) lay their eggs on the ground;
some excavate burrows in the sand or other substrate
(e.g., wedge-tailed shearwater); and others (e.g., red-footed
booby, black noddy) typically build a nest in some kind of
vegetation, although this may place the nest only a foot or
so off the ground. Most seabirds lay a single egg in a
clutch, but the clutch of the brown booby usually contains
two eggs.

On both lava and coral islands, nesting adults are sub-
jected to excessive heat and solar radiation that arc coun-
teracted by physiological and behavioral adaptations. Birds
do not have sweat glands, and both adults and young birds
dissipate excess heat by evaporative cooling. This is
accomplished either by panting or by gular flGttcring, that
is, by rapidly vibrating the throat and floor of the mouth,
thus speeding up blood flow and loss of heat through the
membranes in the floor of the mouth and throat (gular)
areas. Because of the heat stress, the adult bird often must
stand over the egg or recently hatched chick to provide

218

BERQER

shade for it. Chicks also seek the shade of vegetation
when available.

Unfortunately, little precise Information Is available on
the nesting activities of the birds of Enewetak Atoll. No
definitive studies have been made on any of the 12 species
of breeding birds; still awaited Is proof of nesting by the
four species of "possible breeders."

More has been written about the distribution and nest-
ing of the sooty tern on Enewetak than on all other species
together. There now app>ears to be either one or two nest-
ing colonies of sooty terns at Enewetak, and there is con-
siderable historical evidence that the birds shift their nest-
ing sites from island to island. For example, Richardson
(MS) observed a colony of sooty terns on Aej (Olive) dur-
ing February 1959, but none was found at that Islet during
the summer of 1965. Carpenter et al. (1968) estimated a
colony of 16,000 terns on Lujor (Pearl) in 1966, but a
year later the colony was found on Luoj (Daisy) more than
10 miles away. Berger found the only large colony of
sooty terns on Janet Island during the third week of July
1971.

Temme (1979) reports that in November 1977, there
were about 10,000 sooty terns nesting on Aomon (Sally)
in an area cleared only a short time before; when crowded,
the nests are closely packed with 25 to 30 eggs per
square meter. The following March an estimated 5000
adult and Immature birds were there, with a few nests con-
taining eggs. During Temme's visit only a few hundred
additional sooty terns were observed on the 14 other
islands visited. However, during November 1978, an
estimated 29,000 adults, 7800 chicks, and 6700 eggs
were present on Boken (Irene).

In the Hawaiian Islands, William Y. Brown determined
an incubation time of between 28 and 31 days. The young
terns first fly when they are about 57 days old, usually
leaving the island during the day but returning to it to be
fed by the adults at night (Berger, 1981).

Because no data are available for consecutive
12-month periods and for several consecutive years, only
problems for study — rather than conclusions — can be sug-
gested. The shift of nesting jxjpulations of sooty terns from
island to island in different years apparently is partly a
function of vegetation removal and/or rcgrowth. This tern
nests on the ground in open areas, often where scattered
patches of grasses and other low-growing vegetation are
interspersed with open sand or coral.

In the Hawaiian Islands, the brown noddy nests on the
ground; at Enewetak, however, this noddy also often
builds a nest in scrub vegetation (e.g., Scaevola, Pisonia,
Toumefortia) or even in coconut trees. According to
Robert K. Bastian (MS), the nest of the brown noddy at
Enewetak contains sticks, leaves, grasses, feathers, drift-
wood, gravel, algae, sponge, and, characteristically, coral
and shell fragments.

In most parts of its range, the black noddy builds a
bracket-like nest in shrubs and trees, but Atlantic Ocean
populations usually nest on cliff ledges and offshore stacks,
where the birds are safe from mammalian predators. At

Enewetak nests usually are built in trees (especially
Toumefortia and Pisonia). The nests lack the coral and
shell fragments of the brown noddy and usually are com-
posed largely of seaweed and accumulations of feathers
and guano.

A great deal needs to be learned about the breeding
seasons for each species and for the apparent variation in
nesting seasons, especially for the sooty tern. Carpenter et
al. (1968) noted that: "The 1959 and 1962 colonies were
breeding In the March to May period; the 1966 colony was
breeding In the July to September period; In 1967, in June
and July. A satisfactory reason for the apparent variation
on Eniwetok does not seem to exist, though food supply,
precipitation, or even vegetational appearance may be fac-
tors." A thorough study of the sooty tern at Enewetak
would be revealing; in other parts of its range, the sooty
tern nests at 6-, 9-, or 12-month intervals (Ashmole, 1965;
Kikkawa, 1976). An intensive study of banded birds will
be needed to determine the pattern at Enewetak.

The white tern is one of the most Interesting species at
Enewetak and one that occurs on most of the Islets of the
atoll. This tern does not build a nest and usually lays its
single elliptical egg on a bare, horizontal branch of a tree
or shrub, sometimes in the deserted nest of a noddy tern
(Figs. 1 and 2). Carpenter et al. (1968) estimated the
population of white terns to be about 1400 birds for
Enewetak Atoll in 1966; they found the largest concentra-
tion on Liblron Islet. I found the densest populations on
James (Libiron) and Irwin islets during the third week of
July 1971. Many adults were incubating eggs, but there
also were young birds ranging from newly hatched to those
just capable of flight.

THE REEF HERON

The reef heron (Egretta sacra) has a wide range that
includes Korea, Japan, Malaysia, Australia, Melanisia,
Polynesia, and Micronesia. It Is the only heron found at
Enewetak. It occurs In three color phases: white, gray, and
mottled. Most immature birds have the mottled plumage.
Of 57 herons observed by Carpenter et al. (1968), 48%
were white, 28% were gray, and 24% were mottled. Pear-
son and Knudsen (1967) reported a 20:30:50 ratio. Slater
(1971) notes that the "white phase predominates In the
centre of Its range and the grey phase on the periphery."

The reef heron nests on many of the islets of
Enewetak, on or near the ground. The birds apparently
nest singly, with a clutch of three eggs. Carpenter et al.
(1968) found "all stages from eggs to fledgling young"
during June. These herons feed largely on fish and crabs.
Carpenter notes that "the preferred feeding location was
the reef and abandoned landing craft."

REGULAR MIGRANTS AND ACCIDENTALS

Many shorebirds that nest in Alaska and Siberia winter
on islands In the Pacific Ocean. At least 17 species have
been recorded at Enewetak. Some of these have been dis-

AVIFAUNA

219

Fig. 1 White tem egg on a dead branch, James Island, July 27, 1971. [Photo-
graph by A. J. Berger.]

Fig. 2 White terns in flight, Tilda Island, July 24. 1971. [Photograph by A. J.
Berger.]

cussed in detail by Johnson (1973, 1977, and 1979), by
Johnson and Morton (1976), and by Hailman (1979). Many
other species have been recorded in Micronesia (Baker,
1951; Owen, 1977), and some of these certainly will be
found at Enewetak when more intensive field work is done
there.

Several of the wintering shorebirds remain as
nonbreeding individuals during the northern summer breed-
ing season. Johnson and Morton (1976) and Johnson
(1979) discussed this phenomenon for five species. They
thought that such summering birds were first-year birds
that lacked the physiological stimulus for migration. Some

220

BERGER

of the birds that Johnson examined exhibited partial or
even complete breeding plumages although the birds were
biologically immature. "Fat content in summering birds
varies from around 3 to 6 percent of body weight, restrict-
ing them to relatively short flights." Johnson also wrote
that ". . .the sex ratio of plovers was strongly biased
toward males (about 5;1), and apparently balanced in the
other species" (whimbrels, bristlethighed curlews, wander-
ing tattlers, and ruddy tumstones).

Johnson and Kienholz (1975) collected a female short-
eared owl (Asio /lammeusj on Fred islet on July 7, 1973.
Earlier, Amerson (1969) wrote that there were no records
for owls in either the Marshall or the Gilbert Islands. The
subspecies for the collected female was not determined;
hence, the general origin of this accidental visitor is un-
known^

Mailman (1979) discussed several sightings of birds for
which species identification was not possible; these birds
on the hypothetical list include a duck (Anas) and a tern
(presumably of the genus Sterna).

REFERENCES

Amerson, A. B., Jr., 1969, Ornithology of the Marshall and Gil-
bert Islands, Atoll Res Bull No. 127, Smithsonian Institution,
Washington, pp. 1-348.

Ashmole, M. J , and N. P. Ashmole, 1967, Notes on Sea-Birds,
20, Notes on the Breeding Season and Food of the Red-
Footed Booby (Sula sula) on Oahu, Hawaii, Ardea, 55:
265-267.

Ashmole, N. P., 1965, Adaptive Variation in the Breeding
Regime of a Tropical Seabird, Proc Natl. Acad. Sci . 53:
311318.

, 1968, Body Size, Prey Size, and Ecological Segregation in

Five Sympatric Tropical Terns (Aves: Laridae), Svstematic
Zooi. 17: 292-304.

Baker, R. H., 1951, The Avifauna of Micronesia, Its Origin, Evo-
lution, and Distribution, University of Kansas Pub . Mus of
Nat. Hist., 3: 1-359.

Berger, A. J., 1979, Birds and TTieir Habitats on Pacific Islands.
in Literature Review and Synthesis of Information on Pacific
Island Ecosystems, Fish & Wildlife Service, Washington,
D. C, pp. 3-1 to 3-11.

, 1981, Hawaiian Birdlife, 2nd Ed., University Press of Hawaii,

Honolulu.

Brown, W. Y., 1973, The Breeding Biology of Sootv Terns and
Brown Noddies on Manana or Rabbit Island, Oahu, Hawaii,
Ph.D. dissertation. University of Hawaii.
Carpenter, M L., W. B. Jackson, and M. W. Fall, 1968, Bird

Populations at Eniwetok Atoll, Micronesica, 4: 295-307.
Frings, H., and M. Frings, 1959, Observations on Salt Balance
and Behavior of Laysan and Black-Footed Albatrosses in Cap-
tivity, Condor, 61: 305-314.

Gleize, D. A., and D Genelly, 1945, With the Colors, Bull Mass.

Audubon Soc , 29: 221-222.
Goo, C. C, and A. H. Banner, 1963, A Preliminary Compilation
of Tahitian Animal and Plant Names, Hawaii Marine Lab.,
University of Hawaii, Honolulu.
Hailman, J P., 1979, Notes on Birds of Enewetak Atoll, Marshall

Islands, Elepaio, 40: 87-90.
Helfman, G. S., 1973, Ecology and Behavior of the Coconut
Crab. Birgus latro (L.). MS. thesis. University of Hawaii,
Honolulu.
Hines, N. O., 1962, Prouing Ground: an Account of the Radiobio-
logical Studies in the Pacific, 1946-1961, University of Wash-
ington Press, Seattle.
Hutchinson, G. E., 1950, The Biogeochemistry of Vertebrate

Excretion, Bull Amer Mus. Nat. Hist., 96: 1-554.
Johnson, O. W., 1973, Reproductive Condition and Other
Features of Shorebirds Resident at Eniwetok Atoll During the
Boreal Summer, Condor. 75: 336-343.

1977, Plumage and Molt in Shorebirds Summering at

Enewetak Atoll, Auk. 94: 222-230.

1979, Biology of Shorebirds Summering on Enewetak Atoll,

Studies in Avian Biology, 2: 193-205.

, and R J , Kienholz, 1975, New Avifaunal Records from

Eniwetok, Auk. 92: 592-594.
Johnson, O. W , and M. L. Morton, 1976, Fat Content and Flight
Range in Shorebirds Summering on Enewetak Atoll, Condor,
78: 144-145
Kikkawa, J., 1976, The Birds of the Great Barrier Reef, in
Biology and Geology of Coral Reefs, Vol. Ill: Biology 2,
Edited by O. A. Jones and R. Endean, Academic Press, New
York, pp 279-341.
King, W. B., 1967, Preliminary Smithsonian Identification Manual,
Seabirds of the Tropical Pacific Ocean, Smithsonian Inst.
Press, Washington, D. C.
Murphy, R C, 1936, Oceanic Birds of South America. 2

volumes, MacMillan Co., New York.
Owen, R P , 1977, New Bird Records for Micronesia and Major

Island Groups in Micronesia, Micronesica. 13: 57-63.
Pearson, D. L , and J W. Knudsen, 1967, Avifaunal Records

from Eniwetok Atoll, Marshall Islands, Condor, 69: 201-203.
Shreiber, R. W., and D. A. Hensley, 1976, The Diets of Sula
dactylatra, Sula sula, and Fregata minor on Christmas Island,
Pacific Ocean, Pac. Sci., 30: 241-248.
Slater, P., 1971, A Field Guide to Australian Birds, Non-
Passerines, Livingston Publ. Co., Wynnewood, Pennsylvania.
Temme, M., 1979, Bird Populations on Enewetak Atoll,
Mid Pacific Marine Laboratory Annual Report, 1 October
1977-30 September 1978, University of Hawaii, Honolulu,
pp. 67-80.

, 1981, Reproductive Parameters of the Polynesian Rat

(Rattus exulans) in the Northern Marshall Islands, Zeits.
Angewandte Zoologie, 68: 315-338.

, 1982, Feeding Pattern of the Polynesian Rat, Rattus exulans,

in the Northern Marshall Islands, Zeits. Angewandte Zoologie,
69: 463 479
Woodbury, A M., 1962, A Review of the Ecology of Eniwetok
Atoll. Pacific Ocean, University of Utah, Inst, of Biologiceil
Research.

Author Index

Atkinson, M. J., 57

Bastian, R. K, 203
Berger, A. J., 215

Colin, P. L., 27, 91

Duce, R. A., 71

Gerber, R. P., 181

Helfrich, P., 1

Jackson, W. B., 203

Kiste, R. C, 17
Kohn, A. J., 139

Marsh, J. A., Jr., 159
Marshall, N., 181
Merrill, J. T., 71

Ray, R., 1
Reese, E. S., 187
Ristvet, B. L., 37

Vessey, S. H., 203

221

Subject Index

Scientific names of animals and plants and names of people are not included. A name followed by "Island" refers to an
island within an atoll, and atoll names are all followed by the term "Atoll." Subject matter in tables and figure captions is
not included. Code names of nuclear weapons tests are in capital letters followed by the word "test." Military code names of
islands of Enewetak Atoll such as "Alice" are followed by the Marshallesc name in parentheses.

ABLE test, 2
Acanthurids, 153
Acroporids, 113
Aej Island, 31, 218
Aeolidiacean, 147
Ailinglapalap Atoll, 134, 135
Ailuk Island, 135
Air Force Weapons Laboratory,

Albuquerque, 32, 54
Airline of the Marshall Islands, 9
Albatross, 217
Aldabra Atoll, 191, 194
Algae, 31, 43, 93, 101, 140, 143, 151

benthic, 128

blue-green, 128, 140, 142, 143, 148,
153, 164, 191

brown, 141, 144

communities, 93, 110, 113, 115, 116,
125

endolithic, 123

filamentous, 127

green, 100, 116, 121

macroscopic, 14, 153

macro zones, 143

symbiotic, 165, 173, 177
Algal bands

biomass, 99

cover, 153

fragments, 169, 170, 171

lawn, 127

mats, 167

plots, 127

productivity and growth, 164

reef flats, 143

ridge, 12, 30, 31, 41, 43, 105, 141,
142, 143, 145, 148

turf, 140, 141, 142, 143, 145, 146,
169
Alice (Bokuluo) Island, 189
Alinginae Atoll, 27
Alpha helix, 12
Aluminum, atmospheric, 84, 85

American administration, 22, 25

military forces, 20, 199

Samoa, 185
Amino acids, 143
Ammonium 142, 162, 167
Amphinomids, 151
Amphipod, 151
Ananij Island, 12, 31, 100, 103, 110,

180, 203, 209, 210
Anemone fishes, 130, 135
Anomuran, 154
Anthropogenic substances, 82
Anthropologist, 17

Aoman Island, 107, 207, 208, 215, 218
Aquifer, permeable, 54

Pleistocene, 50
Archaeological research, 17
Amo Atoll, 135, 190
Aroclor, 89
Arsenate, 168
Arsenite, 168
Arthropoda, 148
Asian soil dust, 84, 85, 87
Asilomar Conference, 14
Atmospheric chemistry, 71, 82

sea salt, 83, 84, 85

soil dust, 84

tide, 71
Atoll, 189, 197

carrying capacity, 198, 199

ecosystems, 197, 199

environments, 181

formation, 39

geology, 37, 38, 39, 40, 41, 43, 44,
45, 46, 52, 54

morphology, 57, 69

origin and evolution, 39

outer slope, 91

population, 198

soils, 190, 191, 197
Australia, Eastern, 119
Avifauna, 215

Bacteria, 123, 124

BAKER test, 2

Basalt, 187

Beach, 31, 44

Behavior, 7

Behavioral ecology, 194

Benthic environment, 184

fauna, 144, 149

flora, 143

metabolism, 161

plants, 173
Bench substrates, 140
Bijire Island, 31, 44, 207, 210
Biken (Rigili) Island, 30, 31, 54, 58, 126,

187, 194, 200
Bikini Atoll, 1, 2, 6, 12, 17, 22, 25, 27,
37, 57, 58, 61, 62, 66, 68, 69, 99,
121, 135, 191, 198, 203, 209, 211
Bikini people, 21
Billae Island, 31
Bioerosion, 122, 144
Bioherms, 94
Biological communities, 30, 32

zonation, 160
Biomass, 153, 160
Biostromes, 94

Biota, terrestrial, 187, 189, 191
Biotic diversity, 187
Bioturbation, 127, 128, 129, 134
Birds, 154, 187, 191, 193, 195, 215,
216, 217

golden plover, 154

migrant, 215, 218, 219

nesting, 31, 191, 215, 216, 217, 218

reef heron, 203

sea, 189, 190, 191, 197, 215, 217

shore, 154, 219

sooty tern, 205, 210

terns, 190, 195, 216, 217, 218

tropic, 216, 217
Bishop Museum, 16
Bivalves, boring, 122, 123

223

224

SUBJECT INDEX

Blacktip reef shark, 133

Bogairikk Island, 34

Bogallua Island, 216

Bokandretok Island, 31, 82, 103, 130,

197
Boken Island, 31, 34, 36, 134, 187, 200,

218
Boklnwotme Island, 34, 36
Bokoluo Island, 31, 34, 106
Boobies, 217
Borehole, 140
Boulders, 31
BRAVO test, 2
Breadfruit, 31
Bryozoa, 94

Bubbles, wave induced, 181, 182
Buttress zone, 105

CACTUS test crater and crypt, 32, 134,

210
Calcification, 119, 141, 143, 162, 165,

166, 167, 171, 173
Calcium carbonate, 119, 139, 143, 144,
146, 150, 153, 161, 162, 167, 171,
172, 173

producing organisms, 27

production, 161
Calcium crustaceans, 7, 25, 27, 28

transport, 143, 166

uptake, 160, 173
Callianassids crustaceans, 99, 127, 128,

129
Canoes, sailing, 18
Capitellid, 15, 30, 149
Carbonate, 191
Carbon dioxide (CO2), 59, 141, 142, 161,

162, 172
Carbon-nitrogen ratio, 170, 171, 182,

183
Carnivores, 129, 151
Carrion, 197
CASTLE test, 2
Cats, 203
Caves, 116
Cement debris, 31

pavement, 31

ship, 29
Cementation, 190
Cemented deposits, 190
Cerithiid, 150
Cesium, 191
Chaetopterid, 149
Channel, deep, 50, 60, 61, 63
Channels, 110, 114

major, 29

Northern Islands, 107, 199

sand, 114

South, 61, 64, 65, 68, 200
Chemical dissolution, 140
Chemoautotrophic bacterium, 142
Chemoreception, 150
Chinimi Island, 103
Chlorinated hydrocarbons, 89
Chlorophyll, 24, 25, 165, 170, 171

Chop Top reef, 124

Chop, wind produced, 29

Christianity, 20

Christmas Island, 173

Ciguatera fish poisoning, 12, 14

Circulation systems, 57, 69

Cirratulids, 146, 149

Clan, 18

Clay, 191

Climate, 27, 72, 187

marine, 71
Clouds, 189, 210, 220, 270
Cnidaria, 144

Coast Guard LORAN Station, 7, 12, 14
Coconuts, 18, 31, 187, 189, 190, 191

195, 200, 215, 216
Colonial history, 20
Communities, biological, 41
Community metabolism, 173
Copepods, calanoid, 182
Copra, 18, 190, 199
Coral-algal ridge, 139, 140, 145, 148
Coral atoll, 27

autoradiographic, 119

autotrophic, 165

black, 115

boulders, 31, 189

button, 99

cap, 187

colony morphology, 120

deep lagoon, 100

energy requirements, 165

growth, 114, 119, 120, 187

growth gradients, 165

growth rate, 166

heads, 106, 107

head zone, 106, 107

hermatypic, 144

knoll, 30, 44

mucus, 170

nutrition metabolism and growth, 165

pinnacles, 27, 29, 30

pocilloporid, 120

predators, 124

production, 168

radioactive, 6, 15

rubble, 31, 122, 187, 190, 191, 197

sand, 31, 190, 191

skeleton, 122, 123, 124

skeletal damage, 122

stoney, 116, 119

stylasterine, 114

zonation, 140
Cowrey, 146, 150
Crab, 130, 148, 191, 193, 194, 195

behavior, 194, 195, 196, 197

burrows, 195

coconut, 189, 194, 195, 196, 197,
203, 215

copulation, 194

eggs, 194

fertilization, 194

ghost, 154, 205

glaucothoe, 194

grapsid, 148

hermit, 193

land, 187, 189, 190, 193, 194, 197,
215

osmoregulation, 194, 197

physiological ecology, 194

population studies, 196, 197
Craters, 32, 34, 40, 52, 134
Crown-of thorns starfish, 124, 130
Crustacean, 151, 154, 194
Ctenophore, 117
Curlew, 154
Current speed, 60, 63
Cunents, 57, 58, 60, 61, 62, 63, 65, 69,
91, 107

drogues, 61, 63, 65

tidal, 63, 68
Cyclone, 80

Debromoaplysiatoxln, 128
Deep lagoon, 93, 99, 100

benthic communities, 93

prological communities, 93

subtidal habitats, 93
Defense Nuclear Agency (DNA), 7, 12, 14
Denitrification, 28, 172
Department of Defense (DOD), 22, 23, 24
Department of Energy (DOE), 7, 14
Deposition, shallow water, 44
Depths, profile, 115
Desiccation, 189
Detritus, 24, 25, 26, 30, 117, 170, 171,

173, 177, 296
Diademnids, 128
Discovery European, 20
Disease vectors, 197
Diversity, animals, 30

plants, 30, 190
Division of Biomedical and

Environmental Research, AEC, 14,
16
Dogs, 203
Dolphin, 130
Dry season, 71, 73, 74, 75, 80, 81, 187,

189
Dust, concentrations, 84

storm activity, 84
Dye releases, 60, 61
Dynamic heights, 57

topography, 57

Echinoderms, 126, 146
Ecological succession, 190, 201

sustainable yield (ESY) zones, 185
Ecology, 139, 187, 189
Ecosystems, 183, 184, 187, 193, 197,
200, 201

carrying capacity, 197, 199, 201
Eddies, 57, 68
Eggs, bird, 195, 215, 217

fish, 132
Ejecta blocks, 35

trails, 34

SUBJECT INDEX

225

Ekman circulation, 65, 66
Elllce Islands* 210
Elugelab Island. 31
Encrusting mats, 140

zone, 106, 139, 160
Endemic species, 215
Energy, 63, 193, 198
Energy Research and Development -

Administration (ERDA), 14
Enewetak Atoll, 1, 2, 6, 7, 12, 15, 17,
18, 20, 21, 22, 24, 25, 27, 29, 31,
37, 39, 40, 41, 43, 44, 46, 50, 52,
54, 57, 66, 68, 69, 121, 135, 145,
180
bcnthic survey, 93
culture, 17, 18
ecology, 91
economy, 199
genealogies, 18

Island, 7, 20, 58, 60, 63, 68, 71, 81,
91, 103, 118, 119, 120, 121, 124,
125, 132, 140, 141, 143, 145, 147,
148, 151, 153, 154, 162, 187, 198,

199, 201, 203, 205, 207, 209
lagoon, 91, 93, 124, 128
language, 17

Marine Biology Laboratory (EMBL), 6,

12, 16, 159
Municipal Council, 12
people, 7, 12, 15, 17, 18, 21, 22, 23,

25, 191, 198, 199, 200
Radiological Cleanup (ERC), 24, 191,

200, 201
research, 171, 172

Seismic Investigation (EASI), 57

settlement, 17

submersible project, 13
Enjebi Island, 18, 20, 22, 24, 31, 32, 39,
40, 43, 44, 50, 52, 54, 58, 60, 92,
106, 187, 198, 199, 200, 203, 205,
207, 210, 211, 212
Entombment, radioactive materials, 191
Environmental alteration, 199, 216
Enyu Channel, 68, 69
Eocene, 140

Equatorial Counter Current, 135
Erosion, 146
Evaporation, 57
Extracoelentric digestion, 119, 120

Fatty acid esters, 88, 89

salts, 88, 89
Fatty alcohols, 88, 89
Fecal material, 191
Feeding, 149, 150, 153, 216, 217
Ferro-cement barge, 29
Fish, 18, 132, 153, 154, 195
Fisheries potential, 184, 185
Fishery harvests, 184, 185
Fishes, algal feeders, 169

bill, 130

blenny, 153

butterfly, 170

cardinal, 130, 131

carnivores, 169, 170

cleaners, 130

communities, 129, 132

coral polyp feeders, 130, 169, 170

damsel, 127, 132

detritivores, 129

detritus feeders, 169

dog tooth tuna, 129

fauna, 129

feeding activity, 129, 130, 153

food, 129

gobies, 130

grazing, 125, 153

groupers, 130

herbivores, 113, 117, 122, 129, 140,
153, 161, 163, 167, 169, 170

larvae, 132

little tunny, 129

milk, 130

mullets, 129

omnivores, 22, 169, 170

parrot, 113, 122, 125, 143, 153, 169,
170

planktivores, 29, 130, 150, 169, 182

plankton feeders, 169, 170, 171

planktonic eggs, 132

rabbit, 153

rainbow runner, 129

recruitment, 132

scarids, 122, 153

snappers, 130

spawning, 132

surgeon, 125, 143, 153, 169

trigger, 154

tuna, 130

turnstone, 154

wahoo, 130

wrasse, 124

zoogeography, 135
Flies, 197
Flooding, 189
Food habits, fish, 129, 130

humans, 198, 199
Food supplies, 18, 22

web, 198
Foraminifera, 31, 40, 50, 120, 142, 144
Fossils, 140

Fred (Enewetak Island), 220
Funafuti Atoll, 121

Gastropods, 142, 146, 147, 148, 150,
151, 154, 190
shells, 194
Geckos, 190
Genetic effects, 216
Geological perspective, 140, 141
German administration, 18, 20, 190,

199
Gorgonians, 115
Gravels, 190
Grazing, 125, 140, 153
Great Barrier reef expedition, 165, 172
Guano, 190, 191, 217
Gutters, 30

GybenHerzberg lens, 140

Habitat, beach, 154

Habitat selection, rodents, 205

Enewetak, 94, 215
Handicrafts, 199
Hawaii Institute of Marine Biology (HIMB),

16
Hawaii Marine Laboratory (HML), 6
Herbivores, 127, 128, 150, 153
Herbivory, 125
Heron Island, 122
Herons, 218
Herptofauna, 193
Holocene, 141, 172
Holothurians, 126
Homeostasis, 198
Humidity, 27, 72, 73, 189
Humus, 190, 191
Hydroid, 145, 150
Hydrozoans, 121

Igurin Island, 203, 210, 215, 217
Ikuren Island, 6, 31, 113, 187, 190, 191,

194, 196, 197, 200
Indo-West Pacific, 139
Insects, 190, 193, 197
Internal spaces, 164
Intertidal bench, 143

biota, 154

ecology, 139

habitats, 153

substratum, 140

zone, 139, 155
Isaac's Island, 103
Isothermal, 57, 61

Jaluit Atoll, 20, 73, 80, 134, 189

Jamaica, 114

James Foundation, 12

Janet (Enjebi) Island, 216, 218

Japan Meteorological Agency (JMA), 92

Japanese administration, 20, 198, 199

airstrip, 32

military, 20

traders, 198, 199
Japtan Island, 29, 31, 37, 54, 58, 59,

139, 146, 199, 201, 203, 205, 211
Jedrol Island, 29, 31, 130
Jellyfish, 63, 183
Joint Task Force (JTF), 3, 14, 15

Kabelle Island, 217

Kaneohe Bay, Oahu, Hawaii, 163, 172,

184
Kidrenen Island, 31, 60
Kill Atoll, 21

KOA test crater, 31, 34, 40, 41, 134
Kusaie, 27
Kwajalein Atoll, 2, 6, 9, 27, 39, 40, 73,

135, 189

LACROSSE test crater, 34, 43, 134, 170
Ue, 135

226

SUBJECT INDEX

Ugoon, 27, 37, 40, 41, 44, 52, 57, 58,
59, 60, 61, 63, 64, 66, 67, 68, 69,
91, 93, 116, 117, 118, 153, 160,
181

area, 187

bottom, 27, 37, 44, 93

central, 93

circulation, 12, 116

deposits, 121

depth, 91

floor, 160

margin, 30

ocean passages, 27, 29, 30, 93

pinnacles, 63

sediments, 121

slopes, 27

southern, 9, 36, 93

substratum, 15

waters, 60, 116, 163
Lancets, 99
Und, dry, 187

parcels (wato), 18, 19

tenure system, 19
Lawrence Livermore National

Uboratory (LLNL), 31
Lead, 85, 87
Leaf litter, 32, 191
League of Nations mandate, 199
Lebensspuren, 128
Leeward shore, 30
Libiron Island, 218
Lidilbut Island, 31
Liktanur (motor vessel), 9, 15, 16
Limestone, 141, 190, 191
Limpet, 147, 150
Lujor Island, 211, 218
Lojwa Island, 31, 107, 205, 210
Luoj Island, 31, 44, 218

Macrohabitats, benthic, 94

Macruran diet, 151

Majuro Atoll, 12, 17, 22, 135, 199

Makali'i (research submarine), 93, 115,

128, 129
Mangroves, 31, 135
Marcus Island, 27
Mare incognitum. 139, 162
Mariana Islands, 27, 194
Marine communities, 119

environments subtidal, 93

resources, 18
Marshallese bird names, 215

language, 17

people, 215
Marshall Islands, 7, 18, 27, 37, 39, 134,
135

District Administrator, 22

waves, 92
Mass mortality, 154, 155
Medren Island, 1, 3, 6, 7, 29, 31, 32, 37,
39, 43, 45, 46, 93, 100, 103, 118,
119, 132, 144, 199, 200, 201, 203,
205, 207, 211, 212

Pinnacle, 100, 128

Mercury, 87, 88
Metabolic activity, 161

quotient, 142

ratio (CO2-O2), 161
Meteorological data, sources, 81
Meteorology, 27
Mice, 203, 205
Microatoll, 103, 106, 144
Microhabitat, 147, 189
Micronesian Legal Services Corporation

(MLSC), 12, 22
Mid-Pacific Marine Laboratory (MPML),

12, 14
Mid-Pacific Research Laboratory (MPRL),

14, 16, 69, 190
Migration, human, 17
MIKE test crater, 31, 34, 41, 119, 13^

203, 216
Military fortifications, 199
Missionization, 19
Mitrids, 147
Moisture, 189, 190
Mokil Atoll, 20
Mollusks, 122, 146, 151
Monitor lizards, 203, 215
Mortality rate, 146. 154
Mucus floes, 119
Muricids, 147, 151, 154
Mut Island, 6, 114, 187, 190, 200
Muti Island, 139, 141
Mycidaceans, 154

National Climatic Center, 72

National Museum of Natural History, 16

National Oceanographic and

Atmospheric Agency (NOAA), 15
National Science Foundation, 12
Nematode, rat, 209
Neogastropods, 146
Neogenes, 140
Net plankton, 171, 290
Nevada Operations Office, DOE, 7
Nitrate, 25, 59, 68, 191

nitrogen, 162

uptake, 167
Nitrification, 163, 173
Nitrogen, 162, 163, 173

dissolved organic (DON), 162, 163, 171

fixation, 117, 118, 142, 143, 160,
161, 162, 163, 172, 173, 191

flux, 162, 166, 167, 173

metabolism, 163, 173
North Equatorial Cunent, 57, 59, 91,

132, 135
North Pacific Water, 57
Nuclear explosion craters, 200

explosion effects, 12, 190

test sites, 17, 200

weapons testing, 1, 5, 7, 17, 31, 34,
37, 39, 40, 44, 190, 191, 201, 216
Nudibranch, 147, 151
Nukuoro Island, 198
Nutrient concentration, 172, 184

regeneration, 164, 172

transfer, 193
Nutrients, 31, 59, 68, 142, 217

OAK test crater, 134

Oceanography, 57

Office of Defense Programs, DOE, 16

Office of Naval Research, 1, 16

Onotoa Atoll, 1

Operation CROSSROADS, 1, 57, 159

Organic aggregates, 170

carbon, 82, 88, 143, 162, 167, 182,
183

flexes, 183

inputs, 183

lipid, 88

litter, 190

matter, 181, 182, 191, 197

matter dissolved, 31, 171

matter particulate, 171

nitrogen, 162, 163

particles, 181, 182
Organisms, marine, 191

soil, 193, 199

terrestrial, 189, 191
Owl, 20
Oxygen concentration, 161

consumption, sediments, 164, 172

demand, sediments, 164

metabolism, 161

production, 141, 161

Pacific Cratering Experiment (PACE), 23,

24, 210
Pacific-Enewetak Atoll Craters

(PEACE), 40, 41, 44, 46, 50, 52
Pacific Proving Ground, 7
Pacific Science Association, 1
Pandanus, 18, 270
Particulate organic matter (POM), 171

organic nitrogen (PON), 163

C and N, 171

organic carbon (POC), 171
Parry Island, 143
Passage, 27, 29, 30
Passerine species, 215
Passes, 107
PCB, 89
Peptides, 143
Phaeopigment, 171
Phosphate, 59, 68, 163, 168, 191
Phosphorus, 161, 163, 167, 168, 173,
177

cycling, 163, 168, 173

flux, 167, 173

uptake, 168
Photosynthate, algal, 165

zooxanthellae, 167, 168
Photosynthesis, 164, 165
Physical deprivation, 22
Physiographic zones, 139
Phytoplankton, 117, 118, 143, 183
Pinnacle, 100, 101
Pisonia, 187, 190, 191
Plants, 189
Planulation, 120

SUBJECT INDEX

227

Plumage, 220
Plutonium, 60, 191
Pole pinnacle, 100, 124
Political structure, 18, 22, 24, 25
Polychaetes, 123, 145, 146, 149, 150,

151
Ponape, 18, 27, 198
Populations, 164, 197

human, 197, 198, 199, 201

individual, 164, 165, 173
Porifera, 144
Porolithon ridge, 140
Potassium, 191
Precipitation, 73, 74
Predators, 151, 215, 218

fish, 129
Primary producers, 160
Productivity, algal, 164, 165, 169

primary, 141, 142, 143, 161, 164,
165

reef, 184

zooplankton, 182, 183, 184
Promontories, 31
Prosobranch, 151
Protozoa, 144, 183
Pteropods, 117
Pumice, 31

Quarries, 39, 43, 146, 153
Quaternary f)eriod, 141

Radioactive material, 6, 191

Radiocarbon dates, 141

Radiological surveys, 7, 24, 191

Radionuclides, 15, 30, 128, 191, 201

Radionuclide transport, 68

Rafting debris, 31

Rainfall, 27, 37, 57, 59, 73, 74, 80, 189

Rain storm, 154

Rat, nematode, 209

Norway, 197

Polynesian, 197, 205, 206

roof, 197

tapeworm, 208
Rats, 187, 190, 191, 195, 196, 197,
203, 205, 206, 207, 208, 209, 210,
211, 212, 215
Recolonizers, 120
Recycling processes, 28
Redfield ratio, 163
Reef area, 91, 215

back, 58

Carrlbean, 172

communities, 27

currents, 183

destruction, 119

detritus, 182

ecosystem, 1, 5, 27

environments, 181

face, 34

fishes, 116

flat, 2, 3, 6, 26, 27, 28, 29, 30, 31,
32, 34, 41, 43, 44, 57, 58, 59, 64

flat ecosystems, 6, 28, 29, 30, 130,

140, 161, 172, 190, 240

flat rips, 106

fore, 58

front, 60

gross production, 28, 29

growth, 119, 184

heron, 203

leeward, 59, 60, 61, 63

margin, 60

metabolism, 161

net production, 28

particles, 182

patch, 30, 37, 44

perimeter, 57

platform, 139, 148

productivity, 142, 173

rock, 148

rubble, 27

seawater transport, 182

structure, 57, 160

systems, 184

types, 160

windward, 30, 31, 37, 39, 41, 43, 54,
59, 60, 63, 64, 65, 68, 69
Reefs, Pacific, 172
Reentrants, 31
Reference collection, 6
Regenerative spaces, 164, 172, 173
Rehabilitation program, 15
Residence time, 68
Ri Enewetak, 18, 21, 22
RiEnjebi, 18, 20, 21, 22
Rigili Island, 216, 217
Rips, 300
Rock, 191

beach, 31, 43, 44, 148

carbonate, 31

noncarbonate, 280

pavement, 30, 43

substrate, 113
Rodents, 193, 197, 203, 205, 206, 207,
208, 209, 210, 211, 212

adrenal weight, 207, 208

behavior, 206

fighting, 208

food habits, 205

home range, 205

necropsy, 207

parasites, 207, 208

population density, 205, 206, 207,
208, 210

reproduction, 205

wounding, 207
Rongelap Atoll, 27, 217
Rongerik Atoll, 21
Rubble, 30, 31, 103, 148, 191
Runit craters, 32, 34, 134, 200

Island, 31, 32, 40, 43, 44, 50, 63,
115, 191, 205, 210, 211, 212

Sabellid, 149

Salinity, 57, 59, 61, 67, 68, 69

lagoon, 91
Salps, 117

Salt water consumption, 217

Sand, 27, 31, 43, 44, 99, 189, 190

grain size, 58

processors, 126

spits, 58

transport, 57, 58, 189
Scarp, 34

Scavengers, 195, 197, 215
Sclerosponges, 14, 114
Sea anemones, 130, 140, 143, 145

cucumbers, 146, 149, 177

grasses, 134

hare, 128

level, 141, 187

urchin, 99, 113, 127, 128, 145, 146,
150
Sea/Air Exchange Program (SEAREX),

82, 85
Seapens, 116

Seaward lagoon, subtidal habitats, 91, 93,
94

shelf, 31

slope, 31
Sediment-bottomed areas, 30, 37

communities, 70, 172

distribution, 27, 41

production, 21
Sediments, 99, 100, 103, 115, 116, 120,
124, 126, 127, 128, 129

channels, 13

lagoon, 99, 100

patch reaf, 109
SEMINOLE test crater, 134
Shallow lagoon, subtidal habitats, 93
Shark attack, 133, 134
Sharks, Galapagos, 133

gray reef, 133, 134

lemon, 133

reef, 154

research, 7

silvertip, 133

tiger, 133, 134

whitetip reef, 133
Shells, 190
Ship channel, 100
Shrimps, alpheid, 122

ghost, 128, 129, 184
Shrubs, 187, 189
Silicate, 191

Sipunculans, 123, 146, 151
Social organization, 18
Soft bottom, 27

Soil, 18, 31, 39, 40, 50, 187, 189, 190,
191, 199

Arno type, 190

calcareous, 190

contaminated, 200

dust, 84

Jemo type, 190

profiles, 190, 191

removal, 200

Shioya type, 190
Solar energy, 160

radiation, 27, 74 75, 81

228

SUBJECT INDEX

Solution residues, 191

South Medren Pinnacle, 100

Southern ocellation, 71

Southwestern Island, 92

Southwest Passage, 60, 61, 63, 68

Spirorbid, 151

Sponges, 99, 144, 145, 146, 149

clinoid, 122
Spur and groove zone, 31, 39, 41, 43

system, 110
Storm, 103, 120, 154, 190

swells, 124, 125

waves, 189
Stratification, 59
Strontium, 191
Submersible Makali'i. 15
Subsistence foods, 199
Subsurface geology, 27, 32, 39, 40, 41,

46
Subtidal environments, 91, 93
Sunlight, 189
Supratidal fringe, 139
Surf, 58, 60, 63, 64
Suspension and deposit feeders, 149
Swell, ocean, 29, 58
Swells, 93
Syllids, 146
SYMBIOS Project, 12, 159, 161, 162

Tarawa Atoll, 191
Tardigrades, 154
Tattlers, 154
Tectonic movements, 187
Temperature, 27, 57, 59, 61, 69, 140,
187, 189

depth profiles, 91

relationships, 57

substrate, 139
Terrestrial environment, 187, 189, 199,
200

organisms, 189, 197
Tertiary f)eriod, 187
Thermocline, 57
Tidal changes, 29, 37, 54

cycle, 60, 61, 62, 63, 69

range, 60, 139
Tide height, 60

tables, 58
Tides, 58, 61, 62, 63, 69
Tora Shima Island, 27

Tourism, 199
Trade goods, 18

wind belt, 27

winds, 68, 92, 93, 187, 189
Transects, reef flat, 40, 161
Transition zones, 57
Trees, 187
Trophic levels, 160, 168

relationships, 151, 168, 169, 177

transfers, 168, 177
Tropical storms, 27, 60, 124, 187, 189,

190
Tropic bird, 216, 217
Trust Territory of Pacific Island

(TTPI), 7
Tunnel Pinnacle, 100, 125
Typhoon, 27, 93, 187, 189, 190
Typhoon Alice, 154, 189, 190

Ujae Atoll, 17, 27

Ujilang Atoll, 17, 18, 20, 21, 22, 27,

125, 135, 200, 201, 215
Ujilang resettlement, 21
United Nations, 22
United Nations Trusteeship, 7, 200
University of Hawaii (UH), 6, 7, 12
Upwelling, 66, 67, 68, 69
Urea, 167
U. S. Atomic Energy Commission (AEC),

1, 7, 12, 14, 16, 159
U. S. Congress, 24
U. S. Defense Nuclear Energy, 7
U. S Department of Energy, 7, 14
Utirik Atoll, 135

Vegetation, 189, 190, 191, 215, 218

recovery, 190
Vermetids, 146
Vertical mixing, 66

transport, 69
Virgin Islands, 161
Volcanoes, callianassid, 128, 129
Volume transport, 60, 61, 62 63, 65, 69
Von Arx model, 69

Wake Island, 6, 27

Water column, 60, 61, 64, 68, 69

currents, 27, 29

fresh lens, 32, 89

ground, 12, 14, 31, 32, 52, 54, 187,
189

intermediate, 57

isohaline, 57, 61, 68

level, 59

mass, 57

oceanic, 184

Pacific Equatorial, 57

table, 32, 191

temperature surface, 91
Wave action, 27, 43, 92, 140

direction, 57

energy, 57

height, 57
Waves, 30, 43, 57, 58, 69, 92, 93, 94

energy, 57
Weapons testing program, 1
Weather, 27, 71, 72, 189
West Spit, 63, 68, 92, 106
Wet-dry annual cycle, 187, 189
Whimbrels, 154
Wind, 57, 58, 62, 63, 65, 189

data, 57

driven transport, 57, 61, 62, 65, 66,
68, 69

Pacific trade, 71

patterns, 57, 62, 189
Windward buttress zone, 139, 148, 160

platform, 153

reef, 31

shore, 30
Wisconsin glaciation, 187
World War II activities, 18, 25, 31, 189,

215
Wotho Atoll, 17, 27
Wrasse, 124

Xanthid crabs, 148, 150, 151

Zonation, biological, 116

lagoon, 105
Zone, large patch reefs, 107

small patch reefs, 105
Zone of larger coral heads, 121, 160

sand and shingle, 105, 160

small coral heads, 105, 106, 160

stinging coral, 160
Zoogeographic considerations, 135
Zooplankton, 69, 117, 118, 169

diversity, 118
Zooxanthellae, 165, 166, 167, 168, 173

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IS^N 0-87079-580-5

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